The polar bear is a very big white bear. We call it the polar bear because it lives in the Arctic Circle（北极圈） near the North Pole.

    The polar bear lives in snow and ice. At the North Pole there is only snow, ice and water. There is not any land. The polar bear has a very warm coat because the weather is cold in Arctic Circle. You can't find the polar bear easily in the snow because its coat is white.

    A grown-up polar bear is usually about three meters long and weights about 450 kilos. It can stand up on its back legs because it has very wide feet. It can use its front legs like arms. The polar bear can swim very well. It catches fish and sea animals for food. It goes into the sea when it is afraid. People kill the polar bear for its beautiful white coat. The governments of Canada and United States call on that people should stop killing polar bears. They don't want these beautiful animals to die out.

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    One of the hottest places on Earth is Death Valley. It's a desert valley in North America. The animals in Death Valley don't use technology to keep cool. They have special abilities that help them live there.

    During the day in Death Valley, it gets hotter than 100°F. The kangaroo rat rests all day in a hole in the ground. It covers the hole with grass to keep out the heat. The kangaroo rat comes out at night when it is cooler. This little rat is also very good at saving water. It gets all the water it needs from the food it eats. The Sun makes the desert sand very hot. One snake, called the sidewinder, can solve this problem. When it travels, the sidewinder bends (弯曲) its body into a "W" shape. Then it uses the bottom parts of the "W" like legs and walks sideways (向一侧). That way it touches only a little of the sand at a time, so the hot sand doesn't bum the snake. This strange way of moving is how the sidewinder got its name.

    A lot of water in the desert is too salty for most animals to drink. One kind of fish, the desert pupfish, can live in these salty water because it can drink the salt water.

    What will the world of the future be like? There are plenty of people who are happy to give their opinion of what we will be doing in 2050. Here are two predictions about the world of tomorrow.

    When we dream about the future, many like to think of owning a personal flying car. The advantages are obvious. This technology would allow total freedom of movement. We could fly at 480 km per hour, avoiding traffic lights and busy roads. However, some people believe there will be problems with traffic control. If the cars become popular, there is likely to be air traffic jam. Another big problem is mechanical failure. What will happen if the cars stop working? These are problems we must expect if flying cars become a reality.

    Three-dimensional printing is another new technology with exciting possibilities. 3D printers are used to build an object with liquid plastic.

    They build the object layer by layer until it is complete. Car companies already use 3D printers to make life-size models of car parts, and medical companies use the technology to make man-made body parts. As we move into the future, 3D printing will revolutionize the way we shop, the way we manufacture and the way we treat sick people. The disadvantage is that such equipment will be extremely expensive.

    So in the future, we might be able to fly to work or print out new shoes. Although there are some problems to solve before this will be possible, we can certainly dream of a world where technology makes life easier and safer for millions of people.

    Do you know when Earth Day is? It is on April 22nd, and it began in the US in 1970. Earth Day in our country began in the 1990s. Earth Day is a good time for people to think about what they can do to make the Earth clean and beautiful, because now the environment on Earth is getting worse and worse. In fact, there are many things we can do to protect our Earth.

    Plant trees.

    Planting more trees helps to reduce greenhouse gases (温室气体), stop the earth from running away and provide homes for animals.

    Don't waste.

    Do not waste water. Use both sides of paper when you write. Turn off the light when you leave. Take your cup with you. Take your own shopping bags to the supermarket. Stop using the plastic bags. Don't use any disposable things. We should save energy.

    Don't throw rubbish in the streets.

    When rainwater washes over the rubbish, it becomes polluted. The polluted water flows into the rivers and seas.

    Ride your bike or take a bus.

    Too many cars pollute the air and waste energy.

    Remember: Every day is Earth Day.

    We should protect the Earth every day, not just on Earth Day. The Earth is our home. Let's do our best to make the Earth cleaner and more beautiful.

    Height is just one of the thousands of features that your genes(基因) decide. In fact, because you have two parents, your genes provide you a height that usually lands somewhere between the height of each parent. If both your parents are tall, then most probably you will be tall, too. But if you have questions about how tall you're going to be, ask your doctor if he or she can help you find it out.

    But genes don't decide everything. For example, eating an unhealthy diet can keep you from growing to your full potential(潜力). Getting plenty of sleep and enough exercise will help you grow to the expected height.

    No doubt you're wondering how fast you should grow. There's no perfect or right answer. Generally speaking, kids grow about two inches(six centimeters) a year between age 3 and the time when they start puberty(青春期).

    Your doctor will know how your growth has been going over the years, how you've been growing up to this point, and what other changes your body may be going through.

    Don't be scared if you seem to have grown a lot in a very short time. Everyone has a growth spurt(高峰) during puberty. The age for starting puberty is about 10 for girls and about 11 for boys. But it can be earlier or later-between 7 and 13 for girls and between 9 and 15 for boys.

    You'll usually begin to notice that you're growing faster about a year or so after your body starts to show the first changes of puberty.