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.

    You probably know many of the things that you can do with the hundreds of different muscles 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 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 lions' 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. Scientists 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 small leaf from the ground. The nose can also be a powerful tool. An elephant can use its nose to 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.

    The sea looks very beautiful when the sun 1 on it. It can be very terrible when there is a 2 wind.

    The sea is very 3. It covers three quarters(3/4) of the earth. The sea is also very 4 in some places. There is one place, the sea is about 11 kilometers deep there. 5 mountain in the world is about 9 kilometers high. If that mountain was 6 the sea at that place, there would be still 2 kilometers of water 7 it!

    In most parts of the sea, there are 8 fishes and plants. Some live near the top of the sea. Others live deep down. There are also a lot of small living things. Lots of fishes live by 9 them.

    The sea can be very 10. When people go down, the sea becomes colder and colder. Only some men can go down into the deep sea. But, in 1970, five women scientists lived in the deep sea for fourteen days.

    On July 20, 1969, Neil Armstrong became the first person to walk on the moon. The space suit Armstrong wore on his lunar mission has come to symbolize courage and human achievement. In 1971, the Smithsonian's National Air and Space Museum(NASM), in Washington, D.C., acquired (获得) Armstrong's suit. It stood proudly on display for visitors to admire, until there were changes in the suit's appearance. "Over time, the suit started to show signs of deterioration," Lisa Young said. Young is an objects conservator (文物修复员) at NASM. It is her job to preserve(维护)historical and cultural objects.

    "Plastics are tricky to conserve," Young says. They tend to degrade (降解)over time. Neoprene is one type of plastic in Armstrong's suit. The material can turn brittle and break into tiny pieces. This would ruin the space suit. NASM, seeking ways to preserve this historical treasure, removed it from display in 2006.

    What causes plastic to degrade in the first place? "Many things," Odile Madden says. Madden is a scientist in California. She explains that sunlight, water, humidity, and dust can cause plastics to degrade. "Some plastics will break down no matter what we do," Madden says.

    Yet history can still be saved. Scientists have found that storing plastics in cooler temperatures, with lower humidity levels, can slow degradation. Young says Armstrong's suit is currently in storage at a temperature of 63°F and a lower relative humidity of 30%. These conditions will be maintained when Armstrong's suit returns to a museum display case in time for the 50^th anniversary of the moon landing.

    Plastic degradation affects art, too. Artist Claes Oldenburg created False Food Selection in 1966.It consists of plastic food displayed in a wooden box. The plastic food used to look real. Now, some of it has flattened and yellowed. Georgina Rayner is a conservation scientist at Harvard Art Museums, in Cambridge. She says the artwork's wooden box produces an acidic (酸的) gas that eats away at the plastic. This speeds up the degradation process. But Rayner is motivated to conserve this artwork and others like it. "Plastics are a part of our history," she says. "It's important to preserve them so that future generations can understand the journey we've taken."

    Paulina Cortes is a teacher in San Jose, California. One day, she handed out masks (面罩) to farm workers in Lodi. She volunteered to do this because the air quality (空气质量) is still poor after a big fire.

    "I just got home and feel really tired now. Today I spent my day in Lodi handing out masks to farm workers," she wrote on the Internet. "I raised some money. Then I drove through San Jose and all the way to Salinas to buy enough masks I needed. This is important. The story here isn't that I handed out masks; it's that there are hundreds of people who are working in dangerous environments with no protection (防护). And no one even knows about it."

    Paulina also put some photos on the Internet, and they were picked up by the news. Many people were surprised at the news, and they knew what happened to those farm workers for the first time.

    The next day, Paulina added, "I traveled through Lodi and Stockton not just to hand out masks, but to help the farm workers understand the pollution. In Lodi yesterday the workers were picking fruit in such dangerous environments. None of them even knew this. Not too far from our homes, hundreds of people are working in these environments. I will buy more masks and send them to a Stockton farm worker. He helped me hand out masks yesterday."

    Thank you, Paulina, for caring and trying to make a difference.

    The air pollution was so bad in Harbin, in China, on October 21, 2013 that the city had to close roads and schools, and cancel hundreds of flights from its airport.

    News reports said the smog (a mixture of smoke and fog) was so thick that people couldn't see more than 10 or 20 metres in front of them.

    It is also said that people could not even see the person standing next to them. The city's website said: "You can't see your own fingers in front of you."

    The local people said there was a "burning" smell in the air. Many people covered their noses and mouths with scarves or masks.

    The pollution was caused on several factors including: farmers burning off old corn stalks and crop stubble; coal-fired heating systems; and low winds that allowed the smoke to remain over the city.

    The smog had been building up for several days. It became worse when the city turned on the public heating system, which uses coal to heat millions of homes and offices in the city.

    Harbin has a population of more than 10 million people. It lies in northeastern China, where cold winter weather can last up to six months. The main fuel there is coal, because it is cheaper than other types of fuel.

    Air pollution from burning coal can cause health problems like heart disease. During the smog crisis* in Harbin, the number of people taken to the hospital with breathing problems was 30% higher than usual.

    The city government is trying to reduce its use of coal by adding more insulation and better roofs and windows to buildings to reduce heating needs. It has also used cleaner fuel for cars and factories.