teeth will not come
in strong and good, nor will the teeth remain good. This picture
shows how the teeth grow. Notice the gaps between the teeth,
where food may lodge.]

Right in the middle of each tooth is a tiny hollow, or cavity, filled
with a soft, living pulp containing one or two very sensitive nerves;
and when the decay has eaten into the tooth far enough to reach this
nerve pulp, it makes it ache, and then you have _toothache_.

The one and only thing that is necessary in order to avoid all this
decay and breaking away of your teeth, and throbbing toothache, is to
keep the surface of your teeth, and particularly the sides where they
are next one another, clean and smooth and unbroken. And all that is
needed to keep your teeth perfectly clean and smooth is to use your
toothbrush thoroughly after every meal and at bedtime; and then, if
there are any little scraps of food between the teeth that have not
been brushed away, to pick them out gently with a quill toothpick, or
take a piece of silk or linen thread, push it up between the teeth,
and gently saw backward and forward until you have cleaned out the
space between the roots. You should take at least three to five
minutes after every meal and before you go to bed at night to brush
your teeth; and you should brush not only your teeth, but the whole
surface of your gums close up to where they join the lips.

It is almost as important to keep your gums pink and hard and healthy
as it is to keep your teeth clean; and the same thorough brushing will
do both. If the gums are perfectly healthy, they will come well down
over the roots of the teeth, and keep them safely covered right down
to where the glassy outer coating begins, and so leave no gap where
the acids of decay can attack the teeth. Be sure to brush your teeth,
not merely straight backward and forward, but up and down and round
and round as well, both to clean out thoroughly all the grooves and
openings between them and to brush the gums well down over the teeth.

It may seem strange, but one of the best ways to keep your teeth from
growing crooked and irregular is to keep your nose clear and healthy,
so that you can breathe through it freely at all times, both day and
night. Crooked jaws and irregular teeth are more often caused by mouth
breathing than by any other one thing.

You can see why it is best to be careful not to get grit or dirt or
bits of bone in your food, and not to crack nu