he softer bony part of the tooth cannot decay. At the base
of the tooth where the gum joins it the enamel is very thin, so that
the scratch of a pin or other instrument may break it.

Never pick the teeth with a pin or needle. The biting off of thread,
finger nails, and other hard material may crack the enamel. It may
also be softened and eaten away by acid formed where food remains
about a tooth. For this reason a quill or wooden pick or piece of
tough thread, called _dental floss_, should be used to clear the teeth
of food after each meal. Slimy matter collects over the whole surface
of the teeth, and is likely to cause decay in spots unless it is
cleaned off night and morning with brush and water. The chewing of dry
crusts of bread or crackers strengthens the teeth and keeps off decay.

=Why Candy and other Sweets cause the Teeth to Decay.=--A sour
substance called acid usually starts the decay of a tooth by eating
through the enamel. Germs change sugar and other sweets into an acid.
The acid is not made at once. An hour or more is needed for the germs
to grow to form the acid. If, after eating sweet foods, the mouth is
well cleaned, no acid will be formed. Sugar and candy do not,
therefore, spoil the teeth unless it is left sticking about them.

=How to brush the Teeth.=--Every boy and girl should own a toothbrush.
_The teeth should be brushed every night and morning and kept white._
Yellow or gray slimy teeth are very ugly. The teeth should be brushed
on the inside as well as on the outside. It is best to brush the teeth
crosswise for two minutes and then spend another two minutes brushing
the upper teeth downwards and the lower teeth upwards. This prevents
pushing the gum away from the teeth. Plenty of water should be used
with the brush, and a little good powder is helpful once a day.

=How the Dentist can Help.=--Sometimes the milk teeth do not get loose
so that they can be pulled with the fingers at the right time. The
second teeth then come in at one side and may never get straight in
place. They then spoil the appearance of the face and do poor work in
chewing. The dentist should be asked to help straighten the teeth as
soon as they appear crooked.

[Illustration: FIG. 34.--Exact drawing of the teeth of two persons.
Those in the lower picture began to decay over twenty-five years ago and
they were then filled so as to remain perfect. The teeth in the upper
picture began to decay less than ten years ago bu