Thinking back to your pre-adolescent years, you may recall the odd sensation of your baby teeth falling out one by one. Roughly between the ages of six and thirteen, most kids walk around with gap-toothed grins, looking like miniature hockey players until all of their adult teeth come in.

Although the reason why humans have two sets of teeth may seem obvious, scientists have yet to fully concur. The reigning theory is that infant jaws are too small to accommodate adult-sized teeth. Therefore smaller teeth are necessary until the jaw grows to its mature size.

Most kids have all twenty baby teeth, or deciduous teeth, by age two. By the time the average child is in first grade, the jaws and jaw muscles have grown in size and strength, paving the way for the arrival of permanent adult teeth. As the rudiments of adult teeth form in the gums, specialized cells called odontoclasts reabsorb the roots of the baby teeth. By the time the adult teeth start to poke through the gums, the rootless baby teeth are loose and ready to come out.

Adult teeth appear in intervals, beginning with the incisors and first permanent molars between the ages of six and eight. By around age fourteen most of the adult teeth are in place. A few years later, around the age of eighteen, the third and final set of molars, known as wisdom teeth, come in at the back of the mouth. They bring the adult total to thirty-two; until old age, that is, when teeth begin falling out all over again.