Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won't fall easily. Then we say that its center of gravity is low.
    Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).
Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

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We've all heard of mermaids. They are the beautiful half-human, half-fish creatures(生物). They have the head and body of a human and the tail of a fish. We also know that mermaids only exist in fairy tales. But where did all the stories about these creatures come from? If you have ever seen a manatee, then you might have an idea.
Manatees, which are also called sea cows, are mammals(哺乳动物) that live in the sea. Since they are mammals, they have to swim up to the surface to breathe air. Many people believe that the story of the mermaid began with the ancient sailors who saw the manatees putting their heads out of the water to breathe. Because manatees are upright when they do this, it looks like they are standing. If a sailor saw a manatee doing this from far away across the sea, it is possible that the sailor would mistake the manatee for a creature that is half-human, half-fish.
Mermaids are described as having very long hair. This is probably because manatees like to swim up to the surface of the water in areas where there is sea grass. Manatees eat all kinds of plants in the sea, including sea grass. The sea grass around the manatee's head when they eat could create a picture of long hair.

Manatees also have powerful tails that are very similar to the fish-like tail of a mermaid. The sailors probably watched manatees come up for air and then swim back below the surface, using their paddle-shaped tails. This would explain why the sailors thought mermaids had fish-like tails.

    There are many wetlands in China and some of them have become the world's important wetlands. The Chinese Yellow Sea Wetlands are among them. They are in Yancheng, Jiangsu Province. They are home for many different kinds of birds and animals. The world's largest Milu Deer Nature Reserve(自然保护区) is in them. More than700 milu deer live freely there. There are not many red—crowned cranes in the world, but every winter you can see some in the Red—crowned Cranes Nature Reserve in the Yellow Sea Wetlands.

    The temperature in the wetlands is usually neither too high nor too low. There is a lot of rain and sunshine, too. They are usually good places for wildlife. Offering food and home for some special kinds of animals and birds is not the only reason why we need to protect wetlands. Wetlands are important because they also prevent(阻止) flood(洪水). But some people want to change the wetlands to make more space for farms and buildings. This means there will be less and less space for wildlife.

    Luckily, more and more people are beginning to realize(意识到) the importance of wetlands and wildlife. Every year, on February 2^nd, many activities are held to tell people more about wetlands.

    The world's forests become smaller and smaller. People are still cutting down the trees. Why is this happening? There are two reasons—land and wood. People need land for animals and crops. And people want wood. Japan, Europe and North American need wood. There, it is used for buildings or it is made into furniture.

    But most places from the forests are not good for animals or crops. The soil (土壤) in the forests is very thin. When trees are cut down, the soil is washed away by the rain or the wind. Soon there is nothing but sand. Then more land will be needed and more trees will be cut down.

    The world uses about a thousand million (百万) tons of water a day. Water is a human right and everyone should have their share. Yet more than 700 million people around the world have trouble getting clean, safe water.

    Treating wastewater is a good way to provide fresh water for us. And it also helps the environment by keeping waste out of rivers and oceans. 80% of wastewater around the world is not treated at all, and it is running into oceans. But now we have got the technology to treat and reuse the wastewater.

    While 75% of our planet is covered with water, only about 2% is fresh water—that comes from rivers, lakes, ice and snow. The rest, 98% of the water, is in seas and oceans. It is too salty to drink. Then desalination businesses come in. More than 19,000 factories have been built around the world, mostly in coastal countries. They process (加工) more than 92 million tons of water every day. But the technology they use requires a lot of energy.

    Scientists are working to create a less costly technology. They want to produce 20 times more clean water and make sure everyone has enough. But for now, the world still faces each day with not having enough water for everyone.