live without it. Almost every kind of animal and plant in the world must
have oxygen or it will die.
Only about one-fifth of the air is made up of oxygen. Most of the
air—nearly four-fifths of it— is a gas called nitrogen. The rest of
the air is made up of many different gases.
There is also a great deal of water vapor and bits of dust floating in
the air. But these things are not really part of the air itself.
If air is just thin, floating gases, why doesn’t it drift away into
outer space? What keeps the gases from just floating away until they are
gone?
The answer to that is gravity. Earth’s gravity pulls at all the
molecules of air, just as it pulls at you.
The air can no more float off into space than you can!
Turn the glass on its side so the air can escape. You will see bubbles
of air rush out of the glass. Now the glass will fill up with water.
You have proved that air is made up of something that takes up space.
