make the air in the building move about in order to keep it fresh; for
if the air is not fresh, we soon grow tired and sleepy and have
headaches. That is why your teacher keeps the windows open at the top
a foot or so. You can easily see that when there are twenty or thirty
of you breathing out poisons, and each one of you needing about four
bushels of fresh air every minute, the old air ought to be going out
and the fresh air coming in all the time.

[Illustration: VENTILATION

Watch the candle flames. Which way is the air moving, and why?]

That is also why your teacher gives you a recess, so that you can run
out of doors and get some fresh air. Then she can throw open all the
windows and doors and have the air in the room clean and fresh when
you come back again. So when recess comes, don't hang about in the
hallways or on the stairs or in the basement, but run right out of
doors into the playground and shout and throw your arms about and run
races to fill your lungs full of fresh, sweet air and stretch all your
muscles, after the confinement and sitting still. Don't saunter about
and whisper secrets or tell stories, but get up some lively game that
doesn't take long to play, such as tag or steal-sticks or soak-ball,
or duck-on-a-rock or skipping or hopscotch. These will blow all the
"smoke" out of your lungs and send the hot blood flying all over your
body and make you as "fresh as a daisy" for your next lesson.

When you come back into the schoolroom after recess, the air will seem
quite fresh and pure; but unless you keep the windows open, it will
not be long before your head begins to be hot, and your eyes heavy,
and you feel like yawning and stretching, and begin to wonder why the