Let's try doing the following experiment.

 What to do：

 Use string to hang a paper clip or other small object at about chest（胸部）level.

 Pick up a stick，such as a broom handle. Take three or four steps away from the paper clip.

Now close one eye and walk toward the paper clip. Try to touch it with the tip of the stick. If you miss，try again，still keeping one eye closed. Now try it with both eyes open and see if it's easier to do.

 How it works：

 In the experiment，you have to judge how far away the paper clip is. Your brain has several ways of doing this.

 One simple way depends on how much your eye has to focus（聚焦）itself. Using only one eye，most people will find it difficult to touch the paper clip on the first try，but the brain still gets some information about distance—though not much.

 Your brain can judge distance much better when you look at something with both eyes. In fact，your brain has more than one way to use both eyes in judging distance. For example，to see anything clearly，you have to make both of your eyes point at the object. When you look at a nearby object，your eyes turn inward. You can feel this happening if you look at the tip of your finger and move your finger toward your nose.

 Your brain gets information from the eyes about how much they are pointing toward each other. Using that information，the brain can tell how far away any object is，whether it is a basketball or a paper clip.

 A plant is a living thing. It uses its many parts to grow from a small seed to a large plant. Each part helps the plant stay healthy. Plants also need sunlight to grow. They get light from the sun and minerals from the soil. These things help the plant stay alive and grow. 

C

    Paul is a photographer(摄影师). He went to the Arctic to take pictures for NationalGeographic(《国家地理》). Finally, he saw a big and white polar bear swimming to him in the icy water.

    Paul lay down the ice to get a better photo, but the polar bear jumped out of the water and tried to attack him! Luckily for Paul, the ice cracked(破裂), or broke and the bear fell back into the water. The polar bear wasn't so lucky. It couldn't get a meal and had to swim farther away to find food.

    To understand the polar bear's problem, you need to know two things about the Arctic. First, the Arctic is mostly ocean(海洋). Second, ice covers(覆盖) a lot of the Arctic Ocean. In winter, the ice is thick, but in summer, the ice becomes thinner and breaks into pieces. Sea ice is the key to survival for Arctic animals. However, because of global warming, Arctic temperatures are rising and polar ice is melting(融化).

    Polar bears need ice  to hunt(捕捉) seals, their favourite meal. They stand on the ice and wait for the seals to come up for air. As Arctic ice melts, polar bears have to swim farther to find ice floes, or large pieces of floating ice. Sometimes, bears have to swim more than 200 km to find an ice floe. Some bears cannot swim that far and sadly they drown(淹死).

    On his last trip, Paul saw three dead bodies of polar bears. Before global warming threatened(威胁) Arctic ice, a dead bear was a rare sight. Paul is sad to see the dead bodies. Wildlife experts(专家) think global warming will have a terrible effect(影响) on polar bears. In the future, polar ice will continue to shrink and endanger the bears. Some scientists believe 65 percent of the world's polar bears will disappear(消失) by 2050. In other words, polar bears are in great danger.

    In 1969, humans landed on the moon for the first time. Now, many other countries want to go there as well. Why?

    The moon has a lot of minerals (矿物质) and other useful things. We want to find out if we can use them. The moon is our closest neighbor in space. We can use it as a rest station. Then, we can learn more about other planets far away in space.

    The moon is 3,474 kilometers in diameter (直径). It is about one-fourth the size of Earth. If Earth is the size of a basketball, the moon is the size of a tennis ball.

    The moon has much less gravity than Earth. If you can jump 1.5 meters on Earth, you can jump 9 meters on the moon.

    The moon is very quiet. There is no sound there. There are a lot of grey rocks on it. The moon's sky is always black.

    Our moon is about 384,400 kilometers away from Earth. The total length of the Great Wall is about 21,200 kilometers. We would have to put about ____ Great Walls end to end to reach the moon!

    The movie The Martian (《火星救援》) is very popular among space lovers. It shows an astronaut trying to live on Mars.

    Although the idea of living on Mars is great, the reality does not look so good. Mars does not have a friendly atmosphere (大气层) for humans. There is very little oxygen there. And because it is too far away from the sun, it is also very cold there. Sometimes the temperature on Mars can be -133℃. This is much too cold for humans.

    However, scientists say it might be possible to live there even without a good atmosphere. Daytime on Mars is almost the same as Earth's. It means there's enough sunlight to grow plants inside safe buildings. However, they would have to grow in soil from Earth. Mars does not have a strong enough atmosphere to stop the radiation (福射) coming from space, which makes it dangerous to grow things in the soil.

    The radiation also causes problems for our health. One way to live there is to build homes underground or in the rocks on the planet. This can help humans keep away from the radiation and keep warm.

    When could we live on Mars? Scientists are hoping to make it possible to live on Mars. Let's wait and see.