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.
We know that there is air all around the world. We could not breathe without air. Airplanes could not fly without air. They need air to lift their wings. Airplanes cannot fly very high because as they go higher the air gets thinner. If we go far enough away from the earth, we find there is no air.
What is the sky? The sky is space. In this space there is nothing except the sun, the moon and all the stars.
Scientists have always wanted to know more about the other worlds in the space. They have looked at them through telescopes and in this way they have found out a great deal.
The moon is about 384,000 kilometers away from the earth. An airplane cannot fly to the moon but there is a thing that can fly even when there is no air. This is rocket.
I am sure that you are asking, How does a rocket fly? if you want to know, get a balloon and then blow it up until it is quite big. Do not tie up the neck o f the balloon. Let go! The balloon will fly off through the air very quickly. The air inside the balloon tries to get out. It rushes out through neck of the balloon and this pushes the balloon through the air. It does not need wings like an airplane.
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.
Rockets can fly fat out into space. Rockets with men inside them have already reached the moon. Several rockets, without men inside them, have been sent to other worlds much farther away. One day rockets may be able to go anywhere in the space.
练习：
1. What color is the sky?
A) It is blue.
B) It is white.
C) It is grey.
D) It has no color.
2. When an airplane flies too high,
A) the air will be too thin to support its wings.
B) the air will become thicker.
C) the air will exert pressure on it.
D) the air will disappear in no time.
3. A rocket can fly to the moon because
A) it looks like a balloon.
B) it is much lighter than an airplane.
C) it doesnt have wings.
D) it works like an untied balloon.
4. Which of the following statements about a rocket is NOT true?
A) It can fly when there is no air.
B) It can fly without wings.
C) It is made of strong metal.
D) It is propelled by burning gas inside it.
5. A rocket is pushed up into the air when
A) hot gas rushes out of its head.
B) it is powered by gas.
C) hot gas rushes out of its bottom.
D) hot gas rushes out of its neck.