Though your teeth are very hard and glassy looking on the surface, they are much softer and chalkier inside; this glassy coating covers only the _crown_, or free part, of the tooth, which you can see. It leaves the softer inside part of the tooth bare just at the edge of the gums, and particularly between the roots of the teeth, where little scraps of food lodge and decay. When the acids that are formed by the decaying food have eaten away a good deal of the inside of the tooth, the hard, shiny surface is left just like a thin shell; and one day you happen to bite down upon a piece of bone in your food, or try to crack a nut with your teeth, and "crack" goes this brittle shell of your hollow tooth. [Illustration: HEALTHY GUMS MEAN HEALTHY TEETH If the gums are not kept clean and healthy, the second teeth that are getting ready to push out the first 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.