Third Grade - Science - Lesson 19 - Brain and Nervous System
Worksheet activity adapted from Integrated Theme Units published by Scholastic.

Objectives
Explain why the brain is wrinkled.
Brainstorm some of the things brains can do.
Identify areas in the cortex that control various activities.

Materials
Diagram of brain for transparency (attached)
A pillow case, yardstick and a zip-lock bag
Diagram of control centers of cortex for transparency (attached)
Brain worksheet (attached)

Suggested Books
Cole, Joanna. Your Insides. New York: Putnam, 1992. Cole's straightforward text is refreshing. Describing the brain she writes, "It doesn't look like much--just a pinkish gray blob.
Elting, Mary. The Human Body. New York: Macmillan, 1986. On page 49 is a soft and gushy full-page color illustration of the brain with parts labeled and showing wrinkly cortex.
Markle, Sandra. Outside and Inside You. New York: Bradbury, 1991. Contains a brilliantly colored infrared photo of the brain showing the wrinkled cortex.
Meredith, Susan. What's Inside You? London, Usborne, 1991. Includes an illustration of the brain showing control areas.
Parker, Steve. Brain Surgery for Beginners and Other Major Operations for Minors. Brookfield, CT: Millbrook Press, 1995. Steve Parker is a prolific writer and is well known for books in the Eyewitness series. In chapter two he writes, "The brain is terribly busy and bossy. Every second, it sends out orders to body parts such as the heart, lungs and guts, telling them to keep beating, breathing and squirming." Illustrations in this book are in a very different style, a combination of cartoon and Gray's Anatomy.
Rustean, Jean. The Human Body. New York: Dorling Kindersley, 1993. Includes a two-page spread on pages 18-19 that shows the two sides of the brain and what activities each controls.
Smith, Kathy Billingslea and Victoria Crenson. Thinking. Mahwah, NJ: Troll, 1988. This inexpensive paperback, part of the Troll Question Book series on the senses, uses a question and answer format to explain how the brain works, what happens when we dream and what brain damage is. Illustrations are friendly and include a charming dog character.

Teacher's Note
Centuries ago, walnuts were called brain food because of their similarities in shape to human brains (hard shell-skull, wrinkled nut meat-wrinkled cortex and two nut halves-two hemispheres of the brain). If walnuts are available, crack a few as cleanly as possible so the two shell halves can be separated to reveal the brainlike insides. Point out the two sides of the "brain" to the students.

Procedure
Ask: What is the command center of the body? (brain) Where is your brain located? (inside the skull) Remind the students that the skull protects the brain from jostles and bumps. Ask: What do you think the brain looks like? (Accept all answers and write them on the board.) Show the students the transparency of the brain (attached). Have the students make two fists and hold them together. Tell them that their brains are a little bigger than their two fists. Tell them that the brain is pinkish-gray and full of wrinkles. Point out the wrinkles on the transparency. Ask: Why do you think the brain is wrinkled? (Accept all answers.) Have a volunteer come up to the front of the room. Have the volunteer measure the length and width of a pillowcase with a yardstick. Write the dimensions on the board. Have the volunteer measure a zip-lock bag. Write those dimensions on the board. Point out the greater surface area of the pillowcase. Ask the volunteer: Can you fit all this surface area into this much smaller zip-lock bag without folding? Have the volunteer show how it can be done. Ask: What do you see when you look into the zip- lock bag? (a very wrinkled pillow case) Ask the students to imagine how big their brains would be if the wrinkles were flattened out. Remove the pillowcase from the bag and hold it up. Tell the students that with all the wrinkles flattened out, the average brain would be about the size of the pillowcase. Ask: Now why do you think the brain is wrinkled? (to fit a big brain into a small skull)
Ask: What does the word brainstorm mean? (to come up with ideas) Tell the students that you would like them to do something that only humans can do. You would like them to use their brains to think about their brains. The human brain is the only organ that can think about itself! Ask the students to brainstorm and help you make a list of the things human brains can do. The list might include: think, remember, dream, tell the rest of the body what to do, create stories, music, and art, solve puzzles and problems, make decisions, feel anger, happiness, sadness, love, speak, read, write and imagine. Tell the students that the brain also collects information from eyes, ears, tongue, nose and skin. It organizes the information so we can know what is going on around us.
Show the students the transparency of the brain again. Point out that different parts of the brain control different parts of the body. Point to the brain stem or medulla. Tell the students that this part of the brain controls life-support activities such as breathing and heartbeat. Point to the cerebellum. Tell the students that this part of the brain helps coordinate muscles so we can move our bodies. The cerebellum also lets us keep our balance when we move. Ask: What is the biggest part of the brain called? (cerebral cortex) Tell the students that the cortex is what people call gray matter. When a person is especially smart, people might say, "Wow, he or she has a lot of gray matter."
Tell the students that the cortex is the thinking part of the brain. Show the students the transparency of the cortex map (attached). Tell the students that the medulla and cerebellum are deep inside, surrounded by the cortex. Tell them that scientists know what some parts of the cortex do, but they do not know what every part does. There are still many mysteries about the brain that have not been explored. Point out and name the various centers of the cortex on the transparency: thoughts and feelings, speech, touch, movement, sight, hearing, taste and smell. Point out the view of the brain from above. Tell the students that the brain has two halves just as their two fists together make two halves of a whole. One is the right side of the brain, the other is the left side of the brain. Ask the students to point to the left side of their brains. Tell the students that this is the side of the brain they use when they are doing math homework, solving problems, or doing science experiments. It is also the side of the brain they use when they are reading or speaking. Ask the students to point to the right side of their brains. Tell them that this is the side of the brain they use when they draw pictures, make up stories, imagine and dream.
Distribute the brain worksheet and have the students color code the areas of the brain used in the named activites.

Third Grade - Science - Lesson 20 - Brain and Nervous System

Objectives
Describe functions of the medulla, cerebellum and cortex.
Identify which side of the brain controls movement of the left side of the body.
Simulate a neural pathway.
Materials
Brain transparency from Lesson 19
Diagrams of nervous system and neurons for transparency (attached)
2-3 feet of electrical cable available at most hardware stores
Paper sign that says, "Wiggle Big Toe"
Suggested Books
Bruun, Ruth Dowling. The Brain: What It Is and What It Does. New York: Greenwillow, 1989.
Powledge, Tabitha. Your Brain: How You Got It and How It Works. New York: Scribner, 1994. Discusses evolution of the human brain as well as how it functions.
Sandeman, Anna. Brain. Brookfield, CT: Copper Beech Books, 1996.
Simon, Seymour. The Brain: Our Nervous System. New York: Morrow, 1997. Includes fabulous microphotography of neurons and photographs of the brain. Simon's text takes on some very complicated processes and is for advanced readers.

Teacher's Note
With a pair of scissors, cut the shielding or plastic covering of the electrical cable (spinal cord) halfway up the length to reveal multi-colored wires (nerve fibers) inside.

Procedure
Remind the students that last lesson they learned about control centers in the brain. Show the students the transparency of the brain from Lesson 19. Ask: What is the name of the area of the brain that controls breathing and heartbeat? (brain stem or medulla) What part of the brain controls muscle coordination and balance? (cerebellum) What is the thinking part of the brain called? (cerebral cortex, "gray matter") Remind the students that the cortex has two sides and each side has control centers for special kinds of thinking. Tell the students you have a trick question. Ask: If I hold up my right hand, what side of my brain is telling my arm muscles to move? (left side) Tell the students that a surprising fact is muscles on the right side of the body are controlled by the left side of the brain. Ask the students to raise their left hands. Ask: What side of your brain is telling muscles in your left arm to move? (right side) Tell the students that the brain's communication system has a crossover. Messages to muscles on the right side come from the left side of the brain and messages to the left side of the body come from the right side of the brain.
Tell the students that a question you would like them to think about is: If the brain controls the body, how does it let the body know what to do? How does the brain send and receive messages? Ask the students to imagine that they are setting up a headquarters, the command center of an organization. They have workers all over the city that they need to stay in touch with at all times. Ask the students to describe what kind of communications network they would set up to stay in touch with their workers. Possible answers might include use of telephones, beepers, and computers. Tell the students that the body's command center uses a communications network, too. It is called the nervous system. Write this on the board. Tell the students that the nervous system wires up the body so it can communicate with the brain.
Show the students the transparency of the nervous system. Tell them that the main cable for the nervous system is a bundle of nerves called the spinal cord. Write this on the board. Ask: Does anyone remember from studying the skeleton where the spinal cord is found? (inside the backbone) Show the students on the transparency how the spinal cord extends inside the stacked up vertebrae from the brain two-thirds of the way down the backbone. Tell them that the spinal cord is the brain's main connection to the rest of the body. Show the students the length of electrical cable and tell them how it resembles the spinal cord. Show them the bundle of wires or nerve fibers inside the cable. Each of the nerve fibers or wires in the spinal cord connects to many more nerves. They branch off the spinal cord like a tree's branches do from a trunk to smaller and smaller twigs. The network of nerves, the nervous system, is made up of nerve cells called neurons.
Ask five students to come to the front of the room to play the roles of neurons or nerve cells. Have the students stand with arms extending to the sides, each just out of reach of the students' arms on either side. Have the first student in the row be Neuron #1. Explain that Neuron #1 has a message from the brain to the body's big toe. Neuron #1 must pass the message to Neuron #2. Neuron #2 must pass the message to Neuron #3. Neuron #3 must pass to Neuron #4 who must pass it to Neuron #5, stationed at the big toe. Explain that a neuron picks up a message with its dendrite (right hand) and passes a message on with its axon (left hand). Have Neuron #1 indicate which is its dendrite and which is its axon. Give Neuron #1 a sign that says "Wiggle Big Toe" and ask him or her to pass the message on. Explain that Neuron #2 takes the message with its dendrite and passes it on to Neuron #3 with its axon. Have the neurons pass the message down the line from dendrite to axon to dendrite to Neuron #5 at the Big Toe. Tell the students that they have created a neural pathway, a pathway of neurons carrying a message. Point out to the students that there is a gap between neurons. They do not actually touch. Tell them that the message that passes from axon to dendrite is really a spark of electricity. The spark jumps across the gap between neurons. Ask: Have you ever seen a spark of electricity jump between two wires? Tell the students that messages that travel through the nervous system are electrical messages. The electrical signal jumps from neuron to neuron creating message pathways. Tell the students that electrical messages can travel along the neural pathways at speeds of 250 miles per hour. That is a good thing because there are 30,000 miles of nerves in their bodies!
Have the neurons sit down. Show the students the neuron diagram on the transparency. Ask: What does a neuron's shape remind you of? (spider) Point out the dendrites that receive electrical messages and the long axon that sends them. Point out the gap between neurons where electricity jumps across. When the spark jumps we say the neuron has fired. Tell them that the brain is made of 10 billion or more neurons receiving or sending millions of messages within their bodies every second of every day. Tell the students that if you could see the electrical messages traveling in each of them, it would probably look like fireworks sparkling and flashing, their brains lit up with ideas.
Ask the students to think about what it feels like when they get a great idea. Ask them to describe the feeling. Tell them that some people have described it as a light going on in their heads. Ask them to imagine a brain busy with ideas. Is it a bright place? Tell the students that scientists have found that the more we use our brains, the more connections are formed between neurons. This builds neural pathways like the one they built between the brain and the big toe. Tell the students that when they learned to walk and talk, they built neural pathways between their brains and bodies, pathways that neurons travel all the time now. When they learned to read and write, more pathways were built. Learning builds pathways. Tell them that as they grow and learn new things, millions of new neural pathways will light up the command center in their heads Learning how to speak a foreign language, bake a cake, shoot a basket, play guitar, understand how a friend is feeling, solve a problem--all this learning causes neurons to stretch out to other neurons and create new pathways for the brain's communication network. That is why the more we learn, the smarter we are.
Tell the students that in next lesson they will challenge their brains with some memory games and try to answer the question: What is a dream?

Possible Homework
Ask the students to write a description of a dream they remember.

Third Grade - Science - Lesson 21 - Brain and Nervous System
Objectives
Describe how an electrical message travels along a neural pathway.
Identify the work of sensory neurons and motor neurons.
Test for a reflex.
Speculate why the brain dreams.

Materials
Transparency of neuron from Lesson 20
For each group of five students: a tray, ten different objects, a cloth to cover the objects

Teacher Resource
Stein, Sara. The Body Book. New York: Workman Publishing, 1992. This award-winning author offers a comprehensive look at the human body for middle and high schoolers. The last section on the "Biggest Brain in the World" provides an in-depth discussion of neural pathways, memory and dreams.

Procedure
Remind students that last lesson they learned about neurons and built a neural pathway with a message for the big toe. Ask: What is a neuron? (a nerve cell) Show the students the neuron transparency from Lesson 20 and ask them to describe how a neuron passes electrical messages. (It picks up an electrical message, a spark, with its dendrites and passes it along on its axon to the neighboring neuron.) Remind the students that their brains contain 10 billion neurons that carry millions of electrical messages day and night. Challenging themselves to learn new things builds neural pathways in the brain. More neural pathways makes a smarter person.
Tell the students that today they will challenge their brains and get some neurons firing with some memory games. Divide the class into teams of five students. Have the students number 1-10 on a piece of paper and put their pencils down. Tell the students that on each of the trays you will distribute, there are ten objects covered by a cloth. When you say, "go" they are to take off the cloths and look carefully at the objects for only 20 seconds. When you say, "stop" they will recover the tray and then try to make a list of all the objects they remember seeing. Distribute the trays of objects and have the students test their memories. When the lists are finished, make a tally on the board--how many students were able to recall all the objects, nine of them, eight of them, etc. Allow the students 20 seconds to view the objects again and recover the trays. Ask a student in each group to reach under the cloth and remove an object without showing it to the rest of the group. Then have the other students feel under the cloth without looking to try to determine which object is missing.
When the memory games are finished, collect the trays and ask: Can anyone name the five senses we use to gather information about what is around us? (sight, smell, taste, touch, hearing) In the first memory game, which of your senses did you use to learn what objects were on the tray? (sight) Tell the students that their eyes sent information about what was on the tray to their brains to be organized and remembered. Ask: What sense did you use in the second memory game to determine which object was missing? (touch) Tell the students that neurons that carry messages with sense information are called sensory neurons. Write this on the board. Tell the students that neurons that carry messages telling muscles to move are called motor neurons. Write this on the board also. Ask: What kind of neurons did we have at the front of the room last time carrying the Wiggle Big Toe message? (motor neurons) Ask: Have you ever accidentally touched something very hot? What kind of neurons would carry the message telling your brain about what you touched? (sensory neurons) What kind of neurons would carry the message telling your hand to pull away from the heat? (motor neurons) Tell the students that when they touch something hot, sensory neurons quickly send heat and pain messages to the spinal cord. The spinal cord sends these messages on to the brain. The spinal cord does not wait for the brain to react though. It sends a message through motor neurons to the arm muscles telling them to quickly pull away from the heat. This instant reaction is called a reflex. Ask: Why do you think the body has instant reactions or reflexes? (to protect it from injury) Tell the students that the body has other instant reactions or reflexes. Have the students pair up to test a reflex. Have students sit with one leg crossed over the other. Tell their partners to tap the dangling leg just below the kneecap with the side of a hand. Ask: What happens? (The partner's foot jerks up.) Is this a reflex? (yes) Have the partner ask the seated partner to jerk his or her foot up. Ask: Does it take longer? Why do you think it takes longer to respond when you ask? (The message travels farther. It travels from the ears to the brain and then down to the foot.) Have the partners switch places and try the reflex experiment again.
Tell the students that there are things they have learned to do by doing them again and again so that now they can do them almost automatically, without even thinking about it. This is called skill memory. Ask the students to think about how they tie their shoes. Have them tie their shoes step by step in their heads. Ask: Does thinking about how to do it take longer than actually tying your shoe? (yes) Tell the students that skill memory is different from the kind of memory they used to make a list of objects on the tray. Skill memory came with practice. The motor neurons in a pathway got used to firing a certain way until now there is a well-worn neural pathway that lets a person ride a bike, tie shoes or whistle without even thinking about how to do it. Ask: What other things have you learned and practiced so that now they are part of your skill memory? (The list could include dribble a ball, play an instrument, blow bubblegum bubbles, type on a keyboard, etc.)
Ask: What do you think your brain does when you are sleeping? Do you think it slows down at all? (Accept all answers.) Tell the students that there are two kinds of sleep that they experience each night. The first kind happens when their thoughts get fuzzy and they drift off. The second kind of sleep is dream sleep. During dream sleep the brain keeps all the muscles that move the body paralyzed so it cannot move. Meanwhile the brain is very busy, firing neurons and activating the back of the brain, the control center for seeing. Tell the students that if they look at a person who is dreaming they might see their eyes moving back and forth under their eyelids as if they are watching something. Neurons in other control centers are also firing so that sometimes dreamers hear, smell, feel and taste things while they are dreaming. The logical part of the dreamer's brain is busy trying to make sense of the images he or she sees and is trying to arrange them in a story.
Tell the students that dreams are still a mystery. Scientists say that brains may need to dream to stay healthy but they do not know for certain that it is true. Ask: Why do you think that might be true? What do you think dreaming is for? (Accept all answers.) Tell the students that one answer might be that dreaming sorts out the experiences we have had during the day. Dreaming finds a place for experiences and weaves them into our memories. It also might be throwing out the things we don't need, like the brain taking out the garbage at the end of the day. Tell the students that many people keep dream journals, descriptions of dreams that they remember because they believe that dreams can tell them something about themselves or give them ideas or even predict the future.
Suggest that students keep a dream journal at home for a week and see if they can trace things in their dreams to events during the day. Have those students who wish to do so share the dream descriptions they wrote for homework. Suggest collecting them into a class dream journal and have students create illustrations for the journal.

Possible Homework
Ask the students to write an answer to the question: Why do you think the brain paralyzes the body during dreaming? (A possible reason is: If we could move around while dreaming, act on our dreams, we might injure ourselves.)

Third Grade - Science - Lesson 22 - Vision and Optics

Objectives
Describe how the iris controls the size of a pupil and the amount of light that goes into the eye.
Demonstrate how a lens focuses light and produces an upside-down image.
Locate a blind spot.
Label parts of the eye.

Materials
Diagram of eye for transparency (attached)
For each student: Vision Worksheet (attached), hand lens, ruler

Suggested Books
Esbensen, Barbara Juster. Echoes for the Eye: Poems to Celebrate Patterns in Nature. New York: Harpercrest, 1996. This book makes visual connections. It is about a way of seeing that transcends the discussions of how eyes work and returns a sense of wonder to the study of sight.
Lauber, Patricia. What Do You See & How Do You See It? New York: Crown, 1994. This excellent book has a short section on the eye but will be most useful in next month's study of optics.
Parker, Steve. The Eye and Seeing. New York: Franklin Watts, 1989.
Reddy, Francis. Discover Light and Sound. Lincolnwood, IL, 1994. Pages 24 and 25 include very good illustrations of an iris responding to light, parts of the outside and inside of the eye plus a more indepth discussion of how rods and cones work.
Sislowitz, Marcel. Look! How Your Eyes See. New York: Coward, 1977. Illustrated by Jim Arnosky, this friendly book contains simple text on the physiology of the eye.
Smith, Kathy Billingslea and Victoria Crenson. Seeing. Mahwah, NJ: Troll, 1988. This senses series' question and answer approach addresses questions such as: Why do I see colors?, How do I see?, Who needs glasses? and What happens when I get my eyes examined?

Procedure
Tell the students that for the next few lessons they will be learning about how our eyes work and how they enable us to see. Ask: Can we see when it is totally dark? (no) What is one thing that our eyes need in order to work? (light) Ask the students to pair up with a neighbor and look at his or her eyes. Ask: Do you see in the very middle of the eye there is a black center? Tell the students that this is called the pupil. Write this word on the board. Ask: What is another meaning for the word pupil? (student) Tell the students that the pupil in the eye is actually a hole with a clear covering over it. The pupil lets light into the eye like a window lets light into a room. Ask the students to look at the circle of color around the pupil in their neighbor's eye. Tell the students that this circle of color is called the iris. Write this word on the board. Remind the students that they looked in a mirror and watched how the iris worked in a previous lesson (Lesson 17). Tell them that the iris is a kind of muscle. Ask: Do you remember what kind of muscle the iris is? (involuntary muscle) Ask: What is an involuntary muscle? (a muscle that works whether you want it to or not)
Have the students repeat the experiment from Lesson 17. One student will cover his or her right eye, count to 20 and then remove the hand. The partner will observe what happens to the pupil of that eye. Ask: What happened to the pupil that was covered when your partner took away his or her hand? (It got smaller.) Why do you think it got smaller? (The iris made it smaller so less light would go into the eye.) Ask: If we were in a dark room, what do you think our irises would do? (They would open up the pupil to collect more light into the eye.) Tell the students that the iris changes the size of the pupil to control the amount of light that comes into the eye. Ask: What do you think would happen if too much light came into the eye? (It might injure the eye.)
Show the students a tranparency of the eye (attached). Point out that the clear covering over the pupil is called the cornea. (KOR-nee-uh). Light shines through the cornea and the pupil. Point out that behind the pupil is a lens. This lens focuses light that comes into the eye onto the back wall of the eye. The back wall of the eye is called the retina (RE-tuhn-uh). Tell the students that the retina acts a little like a movie screen. Light from outside shines through the cornea, pupil and lens and is focused as a picture on the retina. Point out that the overhead projector is shining a strong light through a lens and projecting a picture on the board. The board is like the retina. The retina is lined with 130 million special light and color receptor cells called rods and cones. Rods and cones send electrical messages along a special nerve, called the optic nerve to the brain. Point out that where the optic nerve meets the retina there are no rods and cones so it is a blind spot.
Distribute a hand lens, ruler and worksheet (attached) to each student. Tell the students that they can find their own blind spots with a simple demonstration. Have the students turn their work sheets over on the blank side. Ask them to measure and draw at the top of the paper, a small O and a small X four inches apart. Tell them to hold the paper at arm's length with the O on the left. Cover the left eye with the other hand. With the right eye, stare at the O and slowly bring the paper closer until the X disappears. The X is now in the blind spot. Ask: Why can't we see the X when it is in the blind spot? (There are no receptor cells--rods or cones--where the optic nerve comes into the eye at the blind spot.)
Have the students hold the blank side of the worksheets toward a window or light source. Suggest that they hold the hand lens close to the paper and move it closer and farther away from the paper until they see a picture come into focus. Ask the students to describe the image they see. Ask the students to look very closely at the picture of the window. Ask: What is unusual about this picture? (It is upside down.) Tell the students that the picture that is focused on the eye's retina is upside down, too. Ask: Why do you think we don't see everything upside down? (When the brain gets the picture, it turns it right-side up.) Tell them that in a later lesson they will learn why lenses turn things upside down.
Have the students label the parts of the eye on the worksheet. When they have finished,
remind them that rods and cones are the names of receptors that line the retina. Cones see colors and fine details, but they need a lot of light to work. They are located mostly in the center of the retina. Draw a circle on the board and label it cones for color. Tell the students that rods work even in faint light but they don't see colors. Rods are arranged around the sides of the retina. Draw a circle around the other circle and label the space between the two circles rods in faint light.

Possible Homework
Students can test the arrangement of their rods and cones by looking directly at something in the dark. Can you see color in the dark? Does this tell you that cones or rods are working? Which works better, looking directly at something in the dark or looking a little to the side of it? What does this prove about the arrangement of cones and rods?

Third Grade - Science - Lesson 23 - Vision and Optics
Eye model and paperclip drop activities adapted from The Science Book of the Senses
by Neil Ardley.

Objectives
Describe why the arrangement of rods and cones in the retina affects how we see things in the dark.
Describe how a model eyeball works.
Demonstrate the benefits of binocular vision.

Materials
To build an eyeball: a spherical, clear glass bowl full of water (a fish bowl is ideal); an 8x8-inch piece of white tissue paper, magnifying glass with handle, two small pieces of modeling clay, flashlight, large index card or piece of card stock with a stick figure cut out of the center, tape
Toy binoculars or two cardboard tubes taped together to make them
For every pair of students: paper cup, five paper clips, a cardboard tube

Suggested Books
Ardley, Neil. The Science Book of the Senses. New York: Harcourt, 1992. Includes several excellent activities dealing with optical illusions.
Asimov, Isaac. Why Do Some People Wear Glasses? Austin, TX: Gareth Stevens, 1993. Asimov discusses how the eyes work and then concentrates on vision problems and how they are corrected.
Dougherty, Paul. The Cheshire Cat and Other Eye Popping Experiments on How We See the World. New York: John Wiley, 1995. A compilation of exhibit information from the popular Exploratorium, the San Francisco science museum. Includes great experiments concerning blind spots, peripheral vision, and persistence of vision which is discussed in the next lesson.
Green, Patrick. Seeing is Believing. New York: Julian Messner, 1996. Although this book in the Amazing Brain series is for older readers, the color photos, array of optical illusions and experiments with depth perception make it appealing for third graders.
Showers, Paul. Look At Your Eyes. New York: Harpercrest, 1992. Part of the Let's Read and Find Out Science series, this book includes a very good explanation of how the eye works along with a simple diagram.
Wexler, Jerome. Everyday Mysteries. New York: Dutton, 1995. Stunning color photos offer new and often puzzling perspectives on everyday objects, such as string and an ear of corn, perspectives that challenge the way we see.
Wright, Lillian. Seeing. Austin: Raintree, Steck-Vaughn, 1995. Contains information on how to take care of eyes.

Procedure
Discuss the results of the rod and cone experiment that the students did for homework. Ask: Were you able to see color in the dark? (no) If you could not see color, which receptor was not working? (cones, because they see color) Were you able to see an object better when you looked straight at it or when you looked a little to the side of it? (to the side) Thinking about how rods and cones are arranged on the retina, the cones in the middle and the rods to the outside, why do you think you could see an object better in the dark when you looked a little to the side
of it? (When you look directly at an object, you are aiming cones from the center of the retina at it. Cones don't work in faint light. When you look to the side of it, you are aiming rods at the object. Rods work in faint light so you can see the object better.) Ask: Would you expect that nocturnal animals, animals that are active at night, would have more rods or more cones in their retinas than we do? (rods) Why? (Rods would let them see better in the dark.)
Tell the students that you would like their help to build a model of an eyeball. Show them the fishbowl full of water. Remind them that this is the shape of an eyeball but that a real eyeball is filled with a jellylike fluid instead of water. Ask: What is the back wall, the movie screen of the eyeball, called? (retina) Have a student come up and tape the tissue paper retina to the back wall of the eyeball. Ask: What focuses the light onto the retina? (a lens) Hold up the magnifying glass and ask: Is this a lens? (yes) Have a student use a piece of clay as a stand and set up the magnifying lens a few inches in front of the bowl. Hold up the card with figure cut out and tell the students that this is what the eyeball will look at. Use the other piece of clay to stand the card a foot or so from the magnifying glass. Have a student come up and shine the flashlight beam through the cut-out figure, lens and fishbowl and onto the tissue paper. Move the fishbowl until the image on the tissue is sharp. Ask the student to describe how this model eyeball works. (Light from an image shines through the lens of the eyeball and through the jellylike fluid to the back wall or retina and makes a picture.) Ask: What do you notice about the picture on the retina? (It is upside down.) Why don't we see things upside down? (because the brain turns the picture right side up)
Move the fishbowl a few inches away from the lens. Ask: What happens to the picture if the retina is a little too far away from or a little too close to the lens? (It looks fuzzy or out of focus.) Tell the students that not everyone's eyes are perfect. Many people have eyes where the lens is a little bit too far or a little bit too close to the retina to focus a sharp image on it. Write nearsighted and farsighted on the board. Tell the students that people who are nearsighted see things clearly when they are close up but usually need glasses to help them get a clear picture of things farther away. Ask: What do you think people who are farsighted might need glasses to see? (Things that are close.) Remind the students that glasses are lenses. They help change the focus point, so pictures will focus clearly on the retina.
Show the students the toy binoculars. Ask: What do binoculars do? (Makes things that are far away look close up.) Write the word binoculars on the board. Tell the students that the word binoculars comes from bi which means two and oculus which means eye--two eyes. Ask the students if they have ever heard the expression: Two eyes are better than one? Ask them to try a game to see if it is true that two eyes are better than one.
Have the students pair up; distribute cups and paperclips to each pair. Tell the students that the object of the game is to hold a paperclip above a cup and drop it in. The trick is, the paperclip dropper must drop it only when his or her partner says so. The partner will be the navigator. He or she will guide the dropper to the cup and say when to drop. First the navigator will guide the dropper using two eyes. Then he or she will cover one eye and try to guide the dropper to the drop site using only one eye. Tell the students to keep score and see if two eyes are better than one. Cups should be placed an arm's length from the navigator. When they have finished one round, have partners switch roles and try it again. Ask students to look at their scores. Were two eyes better than one for hitting their targets? Why do they think two eyes were better than one?
Ask the students to close one eye and pick a spot above the board. Reach out and cover
the spot with a thumb. Now close the other eye instead. Ask: What is the thumb covering now? What happened? (The spot jumped to the side.) Ask: Why do you think the spot looked like it moved? (One eye sees it a little differently than the other one.) Tell the students that each eye sees a slightly different angle of a picture. The brain takes the two pictures and combines them into one. Because the brain has information from these two slightly different angles, we are better able to judge how far away something is than we would with only one eye or one picture. Ask: How would two pictures help the paperclip navigator? (Two pictures would help the navigator see how close or far away the cup is.) Ask the students to hold out their arms with elbows slightly bent. Close one eye and try to bring their pointer fingers together and touch. Now have them try it with two eyes. Tell the students that seeing with two eyes is called binocular vision.
Distribute a cardboard tube to each pair of students. Tell them that sometimes information from two eyes can trick the brain when it tries to combine two pictures. Ask the students to keep both eyes open this time. Give their brains a picture from one eye looking through the cardboard tube. For the other eye, place a hand next to the end of the tube. Now the brain is getting two different pictures--a hand and the view through the cardboard tube. Ask: After a few moments, what do you see? Has your brain combined those two pictures? (The tube appears to go right through the hand.) Tell the students that when the eyes trick the brain it is called an optical illusion. Tell the students that they will be learning about more optical illusions next lesson.