There are many kinds of ants(蚂蚁) in America. One is very strong. People are afraid(害怕) of it and animals are afraid of it, too.

    These ants are often in big groups(群). They eat all the animals on their way. They can kill(杀死) and eat elephants. And they can eat house. Sometimes they can kill people and eat them. When the ants come near, people leave their homes. But people are sometimes happy after the ants pass by (经过), because they can see no insects(昆虫) or snakes(蛇).

    Not many years ago, some farmers were worried because hawks ( 鹰) were taking many of their chicken. The farmers didn't know what to do. Finally they went to the country officials and asked for help.

    "Kill the hawks," the officials said. "We will even pay for them." So the farmers began to think of ways to kill the hawks.

    The farmers killed many hawks. They no longer had to worry about their chickens. But they now had a new worry. Field mice were eating up a lot of the farmers' grain.

    How did this happen?

    Hawks cannot only chickens but also field mice. They eat more field mice than chickens. But the farmers didn't know this. When they killed a lot of hawks, they changed the balance.

    When people move into a new place, they often destroy many wild plants. Often these plants are food for the animals. If the animals can't find enough plants to eat, they will starve or have to leave the place.

    In one part of the USA, for example, the deer there like to eat a certain kind of wild roses. The mountain lions there eat the deer. The number of deer, mountain lions and wild roses doesn't change much if people leave things as they are. But people killed many mountain lions in order to protect the deer. Soon there were so many deer that they ate up all the wild roses. Then the deer began to eat the green leaves of young trees. These trees were important to the farmers. So the farmers thought of ways to protect their trees. Now the deer had nothing to eat, and many of them died. This was another lesson from nature.

    To keep the balance of nature is important for us to remember.

    The Earth is about 4, 600 million years old Man lives on the Earth for only 35, 000 years. But we change our Earth in many ways. Many things we do are good, but some are not good for the Earth.

    Water pollution

    A lot of people, birds and fish die each day because of water pollution. Factories pollute the water. As a result (因此), the water in many rivers and lakes is now very dirty.

    Air pollution

    In big cities, factories, cars and buses are the main causes of air pollution. Many people in cities now have health problems. For example, Mexico City used to be a beautiful sunny capital, but today there are many clouds in the sky. And many people get ill because of the polluted air.

    Soil (土壤) pollution

    In order to have a better harvest (收获), some of the farmers use chemicals too much in their fields. That's bad for the soil.

    Hope for the future

    These problems are very serious for our future, so all of us should do something to protect the environment. If we can stop pollution, our Earth will become more and more beautiful.

    What's a spacewalk? Any time an astronaut gets out of a spacecraft(航天器) while in space, it is called a spacewalk. Astronauts go on spacewalks for many reasons. For example, experiments can be placed on the outside of a spacecraft. This lets scientists learn how being in space affects(影响) different things. By going on spacewalks, astronauts can also fix things instead of bringing them back to the earth to fix.

    When astronauts go on spacewalks, they wear spacesuits to keep themselves safe. Inside spacesuits, astronauts have the oxygen(氧气) they need to breathe and the water they need to drink. To keep the astronauts and the spacecraft safe, the astronauts must leave and go back to the spacecraft through a special door. When on a spacewalk, astronauts use safety  tethers to stay close to their spacecraft. Tethers are like ropes, connect the spacewalkers with the spacecraft. They keep astronauts from floating(漂浮) away into space. Another way astronauts stay safe during spacewalks is by wearing a SAFER. SAFER is worn like a backpack. It helps an astronaut move around in space.

    How do astronauts train for spacewalks? One way is by going for a swim. Floating in space is a lot like floating in water. Astronauts practice spacewalks underwater in a huge special swimming pool. For every one hour they will spend on a spacewalk, astronauts need to train seven hours in the pool. Another way astronauts practice for a spacewalk is by using virtual reality(虚拟现实). It looks and feels just like a spacewalk.

    Today, only three countries have finished spacewalks independently. They are Russia, the United States and China. The first person to go on a spacewalk in the world was Alexei Leonov from Russia. Zhai Zhigang is the first Chinese astronaut to go on a spacewalk. The world record of spacewalks is held by Russian astronaut Anatoly Solovyev. He has been on 16 spacewalks and spent more than 82 hours outside in space.