Look at the centre circles of Diagram (图)A and Diagram B. Which of the centre circles looks larger? In fact, they are exactly the same size.
Then look at the picture on the right. What do you see? A vase? Or two faces？Does the picture change quickly from one to the other again and again? Maybe or maybe not, but you can never see them at the same time.

“What's happening? Is something wrong with my eyes?” You may wonder at what you see. Don't worry. Here is how it goes:
When we look at things, our eyes send messages to our brains and then our brains interpret（解读）the information. However, sometimes our brains interpret the received information in a wrong way. It seems that our eyes are playing a joke on us. This often happens and we call it “visual illusion(视觉错误)”.
Movie makers often make use of visual illusions. They make the objects around actors much smaller or bigger than usual. This makes us believe that the actors are much larger or smaller than they usually are. The movies Jurassic Parkand Honey I shrunk the kids just make use of visual illusions.

    The world itself is becoming much smaller by using modern traffic and modern communication equipment(通讯设备). Life today is much easier than it was hundreds of years ago, but it has brought new problems. One of the biggest is pollution. To pollute means to make things dirty. Pollution comes in many ways. We see it, smell it, drink it and even hear it. Man has been polluting the earth. The more people, the more pollution. Many years ago, the problem was not so serious because there were not so many people. When the land was used up or the river was dirty in one place, people moved to another place. They are slowly polluting the whole world.

    Air pollution is still the most serious. It's bad for all living things in the world but it is not the only one kind of pollution. Water pollution kills our fish and pollutes our drinking water. Noise pollution makes us angry more easily.

Many countries are making rules to fight against pollution. They stop people from burning coal in houses and factories in the city, and from putting dirty smoke into the air.

Pollution by SO₂ is now the most dangerous kind of air pollution. It is caused by heavy traffic. It is sure that if there are fewer people driving, there will be less air pollution.

    The earth is our home. We must take care of it. That means keeping the land, water and air clean. And we must take care of the rise in population at the same time.

B

    You probably know many of the things that you can do with the hundreds of different muscles（肌肉） in your body. But have you ever thought about how the muscles in an animal's body are different from yours? An animal's muscles are made to do the things that are the most important for its life.

    Lions are hunters. They depend on their ability to catch other animals. Lions must be fast in order to catch their prey(猎物). The powerful muscles in their back legs allow them to run at speeds of more than 30 miles per hour. They can also jump a distance of more than 35 feet. The strong muscles in their front legs help them catch their prey.

    Snakes also use their muscles for hunting prey and for movement, but their muscles work in very different ways than lion's do. Because snakes do not have any legs, they use their muscles to push against the ground or other objects to move themselves forward. Their muscles also allow them to eat things that seem much too large. For example, a snake may eat an egg that is larger than the width of its body. How does this happen? The snake's strong neck muscles work together to push the egg down the mouth.

    The elephant's long nose has a lot of muscles. Scientist believe that the nose alone has more than 100, 000 muscles! This is because the elephant uses its nose much in the same way that people use their hands and fingers. Even though the nose is large, it can perform surprisingly tasks. For example, an elephant can pick up a tree by its roots or even to lift a baby elephant.

    The great muscles of these animals seem pretty unbelievable. But the muscles in your body work exactly the way you need them to. After all, it might be fun to be able to jump a distance of 30 feet, but as a human being, there is just not much need for it.

    Workers wear them. Swimmers wear them. Scientists wear them. Woodcutters wear them. People who play sports wear them. Goggles protect the eyes. They keep out water and chemicals. They keep out bits of wood or stone. They might look funny, but goggles do serious work!

    Why do people need to wear goggles? They need to keep the can hurt our eyes. Have you ever gotten a piece of sand or dirt or other small particle in your eye? What happens? It hurts. Your eye begins to fill with tears. Those tears are your body's way of getting the sand or dirt out of your eye. Sometimes this works. Your eye feels much better.

    But what happens if your tears don't work? What happens if the piece of sand has gotten stuck in your eye? Eyes are delicate. They can be scratched or cut easier than skin(皮肤). If the particle is not washed out by tears, you might have to go to the doctor. You could even have problems seeing out of that eye.

    Instead of waiting to see if tears will help, some people use goggles. This can prevent things from getting in their eyes at all. If they use goggles. Their eyes are protected from the things that can cause problems, Goggles are a better way to protect eyes. They are made of plastic or glass, and they are tied around your eye area. Some of them hook behind your ears like glasses. Other goggles have a strap (带子) that goes around your head. Both kinds stay right where you put them until you take them off. That way, you can use both hands to work or play sports, and you don t have to worry about something getting in your eyes.

    Your eyes are safe, and you can get busy working or playing!