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.

    In nature, you may see many insects(昆虫) and animals of different colors. Have you ever wondered why?

    Locusts are super delicious food for birds, but it is not always an easy job for birds to catch them. It is neither because locusts are good jumpers or runners nor because they are smarter than their enemies. The reason is that the colors, of locusts change with the colors of crops(庄稼). When crops are young, locusts look green. But when autumn comes, locusts change to the yellow and brown color as crops do.

    Brown bears, tigers and other animals move quietly through forests. They can't be seen easily by their enemies. This is because they have colors similar to the trees. For the same reason, polar bears that live on a land of snow and ice are white. Butterflies and bees living among the flowers are colorful like flowers. Soil insects are mostly dark colored and they live under the soil(土壤)in a dark and wet environment. However, insects with colors different from plants can easily be found and eaten by others. So in order to survive, they have to hide themselves in the daytime and appear only at night.

    Have you ever noticed some even stranger acts? The ink fish in the sea can send out some very black ink when it faces danger. As the ink spreads over, its enemies will find themselves suddenly in a dark world. At the same time the ink fish immediately swims away. That is how it keeps itself safe though it is not strong at all.

    The kangaroo is the world's largest living marsupial (有袋动物). It was first found on the island of southwestern Australia. The animal was given its name by a famous English tourist in the 18th century.

    The kangaroo may be 7 feet(2.1m) tall when standing upright and the male can weigh as heavy as 200 pounds(90kg). The female (雌性) is smaller and lighter than the male. The kangaroo's head is small, but its ears are large, and its fore feet are really small. The animal has strong back legs and long tail.

    The kangaroo usually covers from 5 to 10 feet(1.5 to 3m) with each jump. The female is usually faster than the male. The mother kangaroo has a pouch (育儿袋) in its abdomen (腹部). The baby kangaroo grows up in the mother's pouch slowly.

    The kangaroo lives on grass and other plants. It is always moving from one place to another, lying down to sleep on the ground during the night. It likes to enjoy the warm sunshine. When it's in danger, it can use its back legs and strong tail to protect itself.

    Answer the following questions with No more than 6 words.