s something--a gas--in the air that makes the flame burn,
and when it has used up all this gas inside the glass, and can't get
any more, it stops burning.

Now you will want to know what this gas in the air is. When we write
about it, we use its nickname, the large capital letter _O_; but its
whole name is _Oxygen_.

Just as the candle flame must have oxygen to keep it burning, so our
cells must have oxygen to burn their impurities, or waste; and if they
don't get the oxygen, and can't burn their impurities, they are
poisoned by them and "go out," or die.

You can see the flame when the candle is burning, but you can't see
the fires that burn in our bodies; there are no real flames at all. I
know it is hard for you to believe that there can be any burning when
our bodies are so wet and damp. But if you can't see it, you can
easily feel it. Blow on your hand. How warm your breath is! Touch your
hand to your cheek. It is quite warm, too. If you run or play hard,
you sometimes become so hot that you want to take off your coat. That
is because your fires are burning faster. The muscles are using more
food and making more scraps to be burned. You breathe faster and
faster till at last you are "out of breath" and feel as if you would
smother or choke. The blood has hard work to bring oxygen enough to
keep the fires going.

After the cells have burned the food scraps, they turn the "ashes" and
"smoke" back into the blood-stream that is always flowing past them.
If the cells did not do this, they would soon smother to death, just
as you could not possibly live in a house without chimneys to carry
off the smoke. And, of course, the blood wants to get rid of this
waste just as quickly as possible.

Part of the waste in the body is liquid, like water, and can flow away
through the blood pipes without needing to be burned. Some of this
watery waste comes out through the skin and stands in beads or drops
upon it. That is the part we call perspiration, or sweat. The rest of
it goes in the blood to another strainer called the _kidneys_, passes
through this as _urine_, and is carried away from the body as the
waste water from the bathtub and the sink is carried away from a
house.

For the "smoke" Mother Nature has still another beautiful plan. She
sends the blood-stream flowing through the _lungs_, where it can send
off its "smoke" and then get fresh air to carry to the cells in the
muscles. When you breathe out, you are sendin