of the kidneys; for, like the liver, they are
sometimes over-worked and do not carry the poisons from the body. You
are helping your kidneys when you drink plenty of fresh clean water
every day, and also when you play or work hard enough to get into a
good perspiration; for, as perspiring carries out some of the poisons,
it leaves less for the kidneys to pour out. You ought to get into a
good perspiration at least once every day, or better, three or four
times, if you wish to keep healthy. The Bible says, "In the sweat of
thy brow shalt thou eat bread"; and you must earn health and happiness
at the same price.

II. SOME FOES TO FIGHT

You have seen that sitting or sleeping in rooms where the air is bad,
or eating the wrong kind of food, or working after you are badly
tired, will poison your blood and hinder the proper working of that
beautiful machine, your body. These poisons are made inside your body,
and you can prevent them by living healthfully and wholesomely. But
there are other poisons, which may get into the blood from outside the
body; and while it is best for you not to think too much about these,
or to worry over dangers that may never come, yet it is well to know
just enough about some of them to be able to keep out of their way, as
far as possible.

The most dangerous form of poisons from outside the body are those
made by the germs of some rather common diseases, which, because you
can "catch" them from some one else who has them, are called
"catching," or _infectious_, or _contagious_.

Some of the germs of these "catching" diseases, like the germs of
typhoid fever, of which we have spoken in connection with our drinking
water, are carried in the water or milk that we drink, or upon the
food that we eat; and one of the worst carriers of germs is the
ordinary household fly.

Not so very many years ago, people did not know that _dirt makes
people sick_. You see, they did not know anything about the disease
seeds (germs) that grow so fast in dirt. They did not like to have
flies about, because flies look so dirty and bite people and crawl
over things and spot them. But nowadays, we will not have flies about
because we know that they have been in dirty places where disease
germs live, and that one little fly can carry thousands and thousands
of these germs on his feet.

Have you ever looked at a fly through a magnifying glass or under a
microscope? If you haven't, try it sometime. You will see tha