A

　　 Scientists have always wanted to know more about the universe.

　　Years ago they knew many things about the moon. They knew how big it was and how far away it was from the Earth. But they wanted to know more about it. They thought the best way was to send men to the moon.

　　The moon is about 384,000 kilometres away from the Earth. A plane cannot fly to the moon, because the air reaches only 240 kilometres away from the Earth. But something can fly even when there is no air. That is the rocket.

　　How does a rocket fly? There is gas (气体) in the rocket. When the gas is made very hot inside the rocket, it will rush out of the end of the rocket, so it can make the rocket fly up into the sky.

　　Rockets can fly out into the space. Rockets with men in them have been to the moon. Several satellites without men in them have flown to another planet much farther away than the moon. One day rockets may be able to go to any place in space.