What is the sky(天空) Where is it How high is it What lies above the sky I am sure that you have asked these questions. They are very difficult to answer, aren’t they
If someone asked you: "What color is the sky " I think 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.
Is the sky full of air I am sure that you have asked this question, too. We know that there is air all around the world. We could not breathe without air. Planes could not fly without air. They need air to lift their wings. Planes 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.
Perhaps we can answer some of our questions now. What is the sky Nothing. Where is it It is all around the world. The sky is space. In this space there is nothing except the sun, the moon and all the stars!
Planes cannot fly very high because as they go higher, the air gets thicker.