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    Imagine the situation. You are driving along a desert or on a mountain. You have no idea where you are. You passed the last house two hours ago. Then your car breaks down. It is night and it is cold. You have no mobile phone. What do you do? Well, next time take a GPS with you. This invention may be able to help you. It is a device(装置) which uses satellites (卫星) to find the user's position(位置). It can find your position to within 20 metres. A GPS cannot start your car, but at least you will know where you are.

    GPS, which means Global Positioning System, is a small radio receiver. It looks like a mobile phone. You can hold it in your hand, or put in your pocket. It is sometimes put into a watch or a telephone. We also find GPS devices in cars, planes, or boats. Some of these devices have electronic maps, so you know where you are. For example, in a city they can tell you the name of the street.

    There are three parts to the Global Positioning System. The first part is the receiver. You can hold it in your hand, or have it fixed into your car, plane, etc. The second part is a group of satellites orbiting the Earth. The receiver contacts at least four of the satellites and calculates(计算) its position. The third part of the system is a network of ground stations. They are all over the world. They control the satellites and make sure they are working well.

    Some people think that in the future the GPS will be as common as the mobile. They are becoming cheaper and more and more accurate(精确的). There are also new uses for the GPS. Perhaps they will become like watches. Everyone will have one and you will never be lost again.

    Plants are very important living things. Life could not go on if there were no plants. This is because plants can make food from air, water and sunlight. Animals and men can't make food from air, water and sunlight. Animals get their food by eating plants and other animals. Therefore, animals and men need plants in order to live. This is the reason why we find so many plants around us.

    If you look carefully at the plants around you, you will find that there are many kinds of plants. Some plants are large while others are small. Most plants are green.

    Some plants have many small leaves. Others have only a few. There are many kinds of leaves. Leaves are of different shapes and colors.

    Leaves are also different in other ways. Some leaves have long thin parts at the end, we call them leaf stalks, some have short leaf stalks, others even have no leaf stalks. The leaf edges are also different. Some leaves have smooth(平滑) edges. Some have a wave-shaped edges. Others have edges like the teeth of a saw(锯子).

    Is there intelligent(有智力的) life on other planets(行星)? For years, scientists said "No." or "We don't know." But today this is changing. Seth Shostak and Alexandra Barnett are astronomers (天文学家). They believe intelligent life exists somewhere in the universe. They also think we will soon contact these beings(人；生物).

    Why do Shostak and Barnett think intelligent life exists on other planets? The first reason is time. Scientists believe the universe is about 12 billion years old. "This is long enough for other planets to have intelligent life," say Shostak and Barnett. The second reason is size——the universe is huge. "Tools like the Hubble Telescope (哈勃望远镜) have shown that there are at least 100 billion galaxies (银河系) ," says Shostak. "And our galaxy, the Milky Way, has at least 100 billion stars. Some planets going around these stars might be similar to Earth. "

    In the past, it was hard to look for signs of intelligent life in the universe. But now, powerful telescopes (高倍望远镜) allow scientists to discover smaller planets--the size of Mars or Earth--in other solar systems (太阳系).These planets might have intelligent life.

    Have beings from space already visited Earth? "Probably not," says Shostak. "It's a long way away. However, intelligent beings may contact us in other way, such as radio signals(信号). "In fact, they may be trying to communicate with us now, but we don't have the right tools to receive their messages. However, this is changing. By 2025, we could make contact with other life forms in our universe and we might help each other.

Vision—phones(可视电话)

    Many inventions are very useful in our daily life. They have greatly changed our life. For example, radios, telephones and televisions are widely used in the world. When you turn on the radio, you can listen. But when you use the telephone, you can not only listen to others but also chat with them. However, you can't see anything at all. The television is much better than both of them. People can watch TV and listen to it, but they can't take part in what they see.

    Today, some people are using a type of telephone called the vision phone. With it two people can see each other when they are talking

    Vision-phones can be of great use when you have something to show the person when you are calling him or her. It may also have other uses in the future. Some day you may be able to call up a library and ask to read a book over your vision-phone. You may be able to do some shopping through it too. Perhaps in the future, the vision-phone will come into wide use in our everyday life.