out through the nozzle.

Our air machine, though it is somewhat different from the bellows in
shape, works in exactly the same way. You remember that you found that
the ribs slant down and can be moved on hinges. Suppose, now, you
place your hands against your ribs and feel the ribs lift as you draw
in a long breath. The air will be sucked into your nose just as it was
into the bellows when you raised the handle. By lifting your ribs, you
have made the chest-cage larger; and the air has rushed into your
nose, down your windpipe, and filled your lungs. If you breathe very
deeply, you will find that your stomach, too, swells out. This shows
that the muscular bottom of the cage, called the _diaphragm_, has been
pulled down, making the cage larger still.

In this chest-cage are millions of tiny air bags that make up the
lungs; and every time you take a breath, the air bags are puffed out
with the fresh air that comes rushing in. By the time you let your
ribs sink again, the air has given its oxygen to the blood, and the
blood has poured its carbon-dioxid smoke into the air bags for you to
breathe out. Nature, with the same bellows, pumps in the oxygen and
pumps out the "smoke."

Now, we breathe into our lung-bellows whatever air happens to be
around us. So we should take care that the air around us is fresh air.

Unless the air were kept in motion by the heat of the sun, causing
breezes and winds, it would become stale and wouldn't do at all for
our lung-bellows to use. The air we breathe must be kept moving and
fresh if it is to make us feel bright and strong and happy. Mother
Nature has given us miles upon miles and oceans upon oceans of this
clear, fresh air to breathe--"all outdoors," in fact, as far as we can
see around us and for miles above our heads. She sends the winds to
move the air about and blow away the dust and dirt; and the sunshine,
you remember, not only to warm the air and keep it moving, but to burn
right through it and kill the poisons. But this brings us to something
else.

You have learned that the air we breathe out would soon smother us,
just as smoke would; and now we will see why. If you blow against the
window pane on a cold day, the glass is no longer clear; and when you
look at it closely, you see that it is covered with tiny drops of
water. This is part of the breath you have just blown out. If the room
is cold enough, you can see your breath in the air; that is, the steam
in your breath