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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.

    Why do plants grow in some places but not in others? Why does some land have so many plants growing on it, while other land has almost no plants growing on it at all?

    To grow, plants need several things. One is warmth. In very cold places almost nothing grows. Plants also need water. In very dry parts of the Earth only a few unusual plants can grow. That's why there are almost no trees or grass in dry deserts.

    Plants must also have a place to put down their roots and grow. They find it difficult to grow on hard land. The town is built(被建立) on hard land. The plants there have only the soil between the cracks of the stones to grow in.

    Another thing plants must have before they can grow is food. What will happen if we try to make things grow on the sandy beach? A few plants, such as grass, will grow in sand, but most plants won't. Even if the weather is warm enough and we water the plants each day, many of them will die because the sand on this beach has almost no food for plants.

E

    Artificial intelligence（人工智能）, or AI in short, is the science of giving computers and machines ability to think and work like humans. AI scientists and engineers have made a lot of cool technologies.

    AI is rapidly catching up with the human ability to read faces. As a result, facial recognition（识别）is used a lot in the world. Chinese police use this technology to identify criminals（罪犯）.Once they caught a criminal when he attended a concert. It also checks the ID of ride-hailing（网约车）drivers and lets people pay for things with a smile.

    AI is also used in self-driving cars, which are believed to be the future of driving. Most drivers may wave or nod at passers-by to let them cross the street, but self-driving cars don't have this ability. Carmakers are testing a light signal system to help them "speak" to humans. The system can show what self-driving cars will do. For example, if a driverless car is yielding（让路）, it will flash two white lights side to side. A rapidly flashing light tells passers-by that the car is about to speed up.

    Service Robots also show you know powerful AI is. Imagine this—you come back home from school and your robot housekeeper is waiting for you at the door. After dinner, he teaches you to play the piano, just like a real music teacher. Such thoughts will soon happen in the real world. In the future, more and more service robots will become good home helpers.

    AI is so powerful that it can help human beings and even replace us in many jobs.

    Mica and her little sister got a tree house this year. "I'm Mica! I love my tree house!" This $28,000 tree house is a gift from their parents. Almost three metres above the ground, the platform (平台) was built between two large trees. "One of the things I really wanted was a little door within the big door. If you go up, you'll find that there is a door handle, which is of the kids' height. And there is an adult size door as well. I told the designer to make it feel like a secret hideout (躲藏处)," said Mica's mother Keenan.

    Bala Sundar and his wife came to America 20 years ago. Unlike Keenan and her husband, who bought a tree house for their daughters, the Sundars got one for themselves. "We wanted to have a regular place where we can relax. So we bought it," he said. With beds, tables and windows, it's a welcoming, comfortable space to spend a summer night.

    Both of these amazing tree houses were created by Dan Wright. He's the founder of Tree Top Builders Inc., and has built more than 400 tree houses since he started the company in 2003. "I think the sense of freedom and independence is why Americans can resonate (共鸣) with tree houses. They get people up off the ground and, for kids, their parents can't reach them anymore. They are up looking down, getting a new perspective on life," said Wright.

    Dinosaurs, one of the most successful groups of animals that have ever lived, evolved(进化) into many sizes and shapes. The word "dinosaur" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles(爬行动物) ".

    Some things to keep in mind about dinosaurs：

    ·Not everything big and dead is a dinosaur. Dinosaurs were a special group of reptiles. Although Pterosaurs(翼龙) were similar, they were not true dinosaurs.

    ·Not all dinosaurs lived at the same time. Different dinosaurs lived at different times. Although you can see Stegosaurus(剑龙) fought against Tyrannosaurus

(暴龙) in some movies like King Kong and Jurassic Park, no Stegosaurus ever saw a Tyrannosaurus. It's because Tyrannosaurus appeared on the Earth about 80 million years after Stegosaurus died out.