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.

    One of the hottest places on the 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.

    The kangaroo rat is one animal that lives in Death Valley. 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 conserving (节约) water. It gets all the water it needs from the food it eats. Some kangaroo rats never take a drink of water in their whole lives!

    The sun makes the desert sand very hot. Animals, like snakes, get too hot if they lie on the hot sand too long. One snake, called the sidewinder, can solve this problem. When it travels, the sidewinder bends (使弯曲) its body into a“W”shape. Then it uses the bottom (底部的) 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 burn 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. All water has some salt in it. As the desert heat dries up pools of water, the salt is left behind. The remaining water in these pools can be much saltier than the ocean. One kind of fish, the desert pupfish, can live in these salty pools because it can drink the salt water.

    China has recently published its first artificial intelligence(AI,人工智能) textbook for high school students in Shanghai to start AI lessons. Forty high schools around the country have become experimental bases for exploring AI education.

    The book, Fundamentals of Artificial Intelligence, includes the history of AI and how the technology can be used in areas such as facial recognition, public security and auto driving.

    Peng Yu is a teacher at the High School Affiliated(附属)to Shanghai Jiao Tong University. He took part in editing the textbook and said it would give guide to students. He said," The book has a good introduction to the area so that students can decide if they want to further study AI at college Students can also find out what they need to learn if they are interested in studying AI. For example, they have to learn Maths because it is the fundamental basis of AI."

    Peng said AI has improved the interest of many high school students already, but he still feels that they can learn much more by taking part in project—based learning programs. Peng is sure that everyone should learn some basic knowledge about AI because it will be part of our everyday lives. It is reported that China needs 5 million AI teachers to keep up with development in the future.

    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 the radiation(辐射) is the greatest danger to spacemen in space. When spacemen are working in space, they are in danger of the radiation from the sun and other stars, which is bad for their health. The harm of the radiation won't be found until their children even grandchildren are born. Some special medicine may work a little, but no really effective medicine has been found so far.

    Space rubbish is also thought to be a great danger to spacemen. It's reported that there were 9,000 man-made things flying in space. About 30% of these are satellites, 10% are spaceships, and the rest are space rubbish. An explosion(爆炸) in space in 1999 made a cloud of 300,000 fragments, each at least 4 mm in size. A small piece of these even knocked a spaceship window and caused some damage(损坏). Scientists are watching and reporting any possible danger all the time. They are working to deal with space rubbish.

    Although space is really dangerous, it interests many people on the earth. In the near future, it may become possible 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 do something to help improve the space environment.