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　　If a man in a spaceship looked down at the Earth, he would see mainly water．He might think that the Earth was one giant ocean with little land．

　　Nearly seven-tenths of the Earth's surface is covered by water．One ocean, the Pacific(太平洋), has an area of 63 million square miles．

　　Many years ago people thought that the land under all this water was flat．Now they know that this is not true．There are as many mountains under the sea sa there are on the land．

　　At first, there was very little salt in the sea．The salt was brought into the sea by rivers．It is possible to measure the age of the earth by measuring the amount of salt in the sea．Scientists can calculate how many years it took for this amount of salt to be carried from the land into the sea．

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　　Dogs like living with people．They are very good friends．They can do many things for people．

　　Some dogs help people to look after sheep, other dogs help them to find the lost children．And some of the dogs can help the blind．

　　Many years ago there was a very good dog．His name was Seeing Eye dog．Now we can see this kind of dogs all the world．They are working for the blind．The Seeing Eye dog is strong and easy to train．He helps the blind to walk from place to place．Before a dog becomes a Seeing Eye dog, he must go to a training school for about three months．First the dog has to learn to sit or stay when he hears the trainer's call．In his next lesson the dog learns to take his trainer across busy streets．The dog has many things to learn．At the end of the training school he must take tests．When he passes the test, the Seeing Eye dog will do things by himself．

　　Now he can help the blind people．The new master may be a man, a woman, or even a child．It takes the dog and his blind master about a month to learn to work and live together．

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