Just as in other animals the teeth appear when needed, and grow into the shape required, so they grow in our own mouths when they are wanted, and of the size and shape required at the time. We are born without any teeth at all; and it is only when we begin to need a little solid food added to our milk diet,--when we are about seven months old,--that our first teeth appear; and these are incisors, first of all in the lower jaw. Then, at average intervals of about three months, the other incisors and the canines appear and, last of all, the molars, so that at about two years of age we have a complete set of twenty teeth. These are called the _milk teeth_. Most animals (_mammals_) have formed the habit of growing two sets of teeth--a smaller, slighter set for use during the first few months or years of life, and a larger, heavier set to come in and take their place after the jaws have grown to somewhat more nearly their permanent size. In our mouths, at about seven years of age, a larger, heavier tooth pushes up behind the last milk tooth,--called the "seventh year molar,"--the milk teeth begin to loosen and fall out, and their places are taken by other new teeth budding up out of the jaw just as the first set did. These take a still longer time to grow, so that the last four of the full set of thirty-two do not come through the gums until somewhere between our eighteenth and twentieth years. These last four teeth, for the rather absurd reason that they do not appear until we are old enough to be wise, are known as the "wisdom teeth." Instead of being, as one might expect, the hardest and longest-lived of all our teeth, they are the smallest and worst built of our molars and among the first of our permanent teeth to break down and disappear. Not only so, but our jaws are so much shorter than they were in the days when man fought with his teeth and knew nothing about cooking and had no tools or utensils with which to grind and prepare his food, that there is scarcely room in them for these last teeth to come through. They often cause a great deal of pain in the process, and may even break through at the side of the jaw and cause abscesses and other troubles. [Illustration: THE REPLACING OF THE MILK TEETH The "second teeth" are shown fully formed in the gums, ready to push out the milk teeth. The wisdom teeth, which appear later, cannot be shown at this stage.--After Gray.]