bedrooms we sleep in and the schoolrooms we study in when we are
children, and the offices or shops we work in when we are grown up.

After you have your lungs and your blood well filled with air, either
by walking briskly to school or by chasing one another about the
school playground, you will suddenly hear the bell ring, and you march
indoors and sit down at your desks. Here, of course, the air cannot
blow about freely from every direction, because the walls and doors
and windows are shutting you in on every side. The room, to be sure,
is full of air; but if the doors and windows are shut, this air has no
way of getting outside, nor can the fresh, pure air out of doors--even
though it be moving quite fast, as a wind or a breeze--get inside.

[Illustration: A CLASSROOM ALMOST AS GOOD AS THE OUT-OF-DOORS

Notice the windows open top and bottom, and the high windows
under the roof. Why are these good?]

We must let the fresh air come in and the stale air go out. This is
one of the things that windows are for; and this is why they are hung
upon pulleys and made to slide up and down easily. Of course, even
when the windows are not open, they are letting in light, which, you
remember, is a deadly enemy to germs and poisons.

Bright sunlight is best for purifying the air of a room, but even
ordinary daylight has a good deal of germ-killing power. Therefore, a
room that is well lighted is not only much pleasanter to live in, but
much healthier, than one that is dull and gloomy. You see why we need
plenty of windows and doors: we must let in the breezes and the
sunshine, and let out the poisons and the dirt. Then, too, we must