see it like the gas that comes out of a boiling kettle or from the
whistle of a locomotive, and which we call _steam_. This is simply
because air is so pure that it has no smell, and is so perfectly clear
that we can see right through it. Almost the only way that we can
recognize it is by feeling it when it is moving. But it is a very real
thing for all that; and, like sunshine and food, is one of the most
important things in the world for us.

What is it that air does in the body? We must need it very much, for
we die quickly when we cannot get it: it takes us only about three
minutes to suffocate, or choke to death, if we can't get it.

You remember that the blood is pumped out from the heart, all through
the body. Everywhere it goes,--to the feet and the hands and the
head,--it is carrying two things: food that it has sucked up from the
food tube, and hundreds and hundreds of tiny red sponges called red
_corpuscles_. These little sponges are full of air which they sucked up
as the blood passed through the lungs. When we stop breathing,--that
is, taking in air,--the little red sponges of course can't get any air
to carry to the different parts of the body.

The body is made up of millions of tiny, tiny animals, called
_cells_,--so tiny that they can be seen only under a microscope. Each
of these cells must have food and air, just like any other animal.
They eat the food the blood brings to them, and they take the air from
the red corpuscles in the blood. With the air as a "draft," they burn
up the waste scraps, as we burn scraps from the kitchen, in the back
of the stove.

Suppose you light a candle and place it under a glass jar and watch