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    For many years, scientists have said that the earth is getting warmer. Now they have found something new—hot cities! In the southern US, they have found that cities become very hot in summer. For example, the city of Atlanta in Georgia has roof(屋顶) temperature of up to 50^oC while it is 27℃ in the streets! At night, the outside of buildings stays so hot that the heat of the city causes storms over the city!

    China also has the same kind of problems. Every year more farmland is used for factories or offices. More housing is needed, too. As people become richer, they buy more cars. New roads are needed and new car parks are built in the city centers. This all makes cities hotter.

    It isn't easy to change the situation. City planners say that we should plant more trees in the middle of cities. Every new street should have trees on both sides, they say. Trees make the temperature lower, so we should have more trees in our parks and squares. We should also paint our roofs white. If we do this, they do not become so hot. Every roof in a hot and sunny country should have solar(太阳的) roof panels. The electricity from these can be used to run the air conditioners (空调)in the buildings.

    People continue to cut down forests around the cities. This makes the problem worse. Cities are growing faster and faster. By the year 2025, four fifths(五分之四) of the population will be living in cities. If we go on like this, there will not be enough farmland to feed everyone in the world.

    Height is just one of the features 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 them 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.

    It's natural that you're wondering how fast you should grow. There's no perfect or right answer. Generally speaking, kids grow about 6 cm a year between age 3 and the time when they start puberty.

    Your doctor will know how your growth has been going over the years. 2 cm here and 3 cm there make a little difference to the height you're at now. How have you been growing up to this height? What other changes may your body be going through?

    Don't be amazed 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 9 and 15 for boys.

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

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    Now, it seems that artficial intelligence （人工智能）is becoming more and more popular in our life, and it has greatly influenced the ways we live, work and play. Can you imagine that you can play table tennis with a robot one day?

    Recently, a scientist has invented a special robot that can be a table tennis coach. That means people can practice table tennis not only with humans but also with a robot.

    Named Forpheus, the smart table tennis robot is quite special. It can study the movements of human players and the speeds of the ball. In this way, it can exactly judge (判断) the player' skill levels (水平). Then it uses the information to change the ways to play. If the player are just beginners, Forpheus will play in a slow and easy way, but if the players are better ones, the robot will play in a faster and more difficult way. What's more, while playing, it encourages players to try their beat with words like "Good job!", "Come on!" and so on. At the same time, it gives some good advice to improve their skills. So it is both a good partner and a clever coach.

    Science and technology is developing quickly these days and it's hard to imagine what will happen in the future. "In the next 20 years, it will be possible that one robot teaches another to play table tennis or even invents another one!" Takuoya, the inventor of the robot, said.

    It is important for us to look after the Earth well. The Earth gives us a lot of things. We also give the Earth a lot, but some of the things are not good.

    In nature, when something dies, other animals and plants get food from it. Every animal or plant provides food for other animals or plants. However, animals can't get food from many of the things that we"give" the Earth. Animals and plants can't cat metal, plastic or glass. These things will stay in the ground for many, many, many years.

    Some rubbish is very dangerous for plants and animals. In some places, many animals live together. One animal makes food for many other animals. If we throw rubbish and chemicals into the water, the plankton（浮游生物）can die. If there isn't any plankton, many animals have nothing to eat.

    So what can we do? Don't leave any rubbish in the countryside! Don't make so much rubbish.

    Have you ever dreamed about a trip to the space? Seeds (种子) of some vegetables and crops are a lot luckier than you, as they have travelled to space and back.

    China is among the three countries in the world that carry out agricultural research (农业研究) in space. The others are Russia and the US.

    Since 1987, China has sent about 1, 000 species of seeds into space.

    These include green peppers, tomatoes, cucumbers (黄瓜), carrots, lettuce and wheat. Even on China's first manned spacecraft Shenzhou—V, there were some seeds from Taiwan.

    "Conditions in outer space such as radiation (辐射) and low gravity can cause big changes in the seeds," said Chinese researcher Jin Xing.

    But not all seeds are better after going to space. After returning to the Earth, the best seeds are selected. These seeds produce (生产) much more and also grow faster than normal (正常的) seeds

    As space science develops, man has learned more and more about space. Space is not only amazing but also dangerous. While working in space, spacemen are facing danger as well as success.

    Scientists have found out that radiation(辐射) is the greatest danger to spacemen in space. When spacemen are working in space, they are in danger of radiation from the sun and other stars, which is bad for their health. The damage(危害) done by radiation won't be discovered until their children or even grandchildren are born. Some special medicine may work a little, but no really helpful medicine has been.

    Space junk is also thought a great danger to spacemen. It's reported that there are 9, 000man﹣made things flying in space. About 30% of these are satellites, 10% are spaceships, and the rest are space junk. An explosion(爆炸) in 1996made a cloud of 300, 000 fragments, each at least 4mm in size, and even a small piece of these knocked against a spaceship window and did some damage. Scientists are watching and reporting any possible danger all the time. They are working on helpful ways to do with space junk.

    Although space is really dangerous, it interests everyone on the earth. In the near future, it may become usual for people to spend a few days in a space hotel. We may even choose to leave the earth and live in space in a few years. So we should care for space and do something to help improve the space environment.