Suppose that you prick a hole in the ball and squeeze it. It is now good for nothing. But the rubber is all there. Why is the ball good for nothing?
It is because the air which filled the ball and made it round has escaped. The ball is of no use unless you can keep it full of air.
Perhaps you think that air does not weigh anything. But it does weigh something, though very little, and its weight is well known.
You can not see air, but you can sometimes feel it. You can not feel it while it is still if you are still at the same time. You can feel it only when it is in motion. When the wind blows upon you, it is air in motion that you feel.
When you fan yourself, the air strikes upon your face, and you feel it. When there is a gust of wind, the air comes against you just as a wave of water does.
Sometimes we say the wind blows very hard or very strong. This is when the air moves very fast. When there is only a gentle breeze, the air is moving very slowly.
When the air moves very fast, it sometimes does a great deal of harm. It roots up trees and blows down houses.
The air is clear, like glass. That is, it lets you see through it. But when you look up through the air, you see that it is of a blue color. You call the blue air the sky. The sky is the blue air that surrounds the earth.
—Worthington Hooker.