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.

    Scientists are trying to make the deserts into good land again. They want to bring water into the deserts, so people can live and grow food. They are learning a lot about the deserts. But more and more of the earth is becoming desert all the time. Scientists may not be able to change the desert in time. Why is more and more land becoming desert? Scientists think that people make deserts. People are doing bad things to the earth. Some places on the earth don't get much rain. But they still don't become deserts. This is because some plants are growing there. Small green plants and grass are very important to dry places. Plants don't let the hot sun make the earth even drier. Plants don't let the wind blow the dirt away. When a little bit of rain falls, the plants hold the water. Without plants, the land can become a desert much more easily.

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