If someone asked you, What color is the sky? I expect that you would answer, Blue. I am afraid that you would be wrong. The sky has no color. When we see blue, we are looking at blue sunlight. The sunlight is shining on little bits of dust in the air.　　What is the sky? The sky is space. In this space there is nothing except the sun, the moon and all the stars.　　The moon is about 384,000 kilometers away from the earth. An airplane cannot fly to the that can fly even when there is no air. This is rocket.　　This is how a rocket works. It is not made of rubber like a balloon, of course. It is made of metal. The metal must not be heavy but it must be very strong. There is gas inside the rocket which is made very hot. When it rushes out of the end of the rocket, the rocket is pushed up into the air.　　练习：　　1. What color is the sky?　　B) It is white.　　D) It has no color.　　A) the air will be too thin to support its wings.　　C) the air will exert pressure on it.　　3. A rocket can fly to the moon because　　B) it is much lighter than an airplane.　　D) it works like an untied balloon.　　A) It can fly when there is no air.　　C) It is made of strong metal.　　5. A rocket is pushed up into the air when　　B) it is powered by gas.　　D) hot gas rushes out of its neck.