what will happen. The flame will become weaker and weaker, and at last
it will quite go out. You might think at first that the wind blew it
out; but how could the wind get through or under the jar? No, the
glass keeps all the outside air away from the flame; and that is just
the reason why it does go out. Unless it has fresh air, it cannot
burn. There is 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.