Humans are diphyodont (pronounced: dy-fy-uh-dant), meaning that they develop two sets of teeth. The first set of teeth, the deciduous (pronounced: duh-sid-you-wus) teeth are also called the milk, primary, temporary, falling-off, or baby teeth. These teeth begin to develop before birth, push through the gums between the ages of 6 months and 1 year (this process is called eruption), and usually start to fall out when a kid is around 6 years old. They are replaced by a set of 32 permanent teeth, which are also called secondary or adult teeth.

Although teeth aren't visible at birth, both the deciduous and permanent teeth are forming beneath the gums. By the time a child is 3 years old, he or she has a set of 20 deciduous teeth, 10 in the lower and 10 in the upper jaw. Each jaw has four incisors, two canines, and four molars.

The deciduous teeth help the permanent teeth erupt in their normal positions; most of the permanent teeth form just beneath the roots of the deciduous teeth above them. When a primary tooth is preparing to fall out, its root begins to dissolve. This root has completely dissolved by the time the permanent tooth below it is ready to erupt.

The phase during which permanent teeth develop usually lasts for about 15 years as the jaw steadily grows into its adult form. From ages 6 to 9, the incisors and first molars start to come in. Between ages 10 and 12, the first and second premolars, as well as the canines, erupt. From 12 to 13, the second molars come in. The wisdom teeth (third molars) erupt between the ages of 17 and 21. Sometimes there isn't room in a person's mouth for all the permanent teeth. If this happens, the wisdom teeth may not come through at all. Overcrowding of the teeth is one of the reasons people get braces during their teenage years.

What Do the Mouth and Teeth Do?

The mouth and teeth play an important role in digesting food. Food is torn, ground, and moistened in the mouth. Each type of tooth serves a different function in the chewing process. Incisors cut foods when you bite into them. The sharper, longer canines tear food. The premolars grind and mash food. Molars, with their points and grooves, are responsible for the most vigorous grinding. All the while, the tongue helps to push the food up against our teeth.

As we chew, the salivary glands secrete saliva, which moistens the food and helps break it down further. As well as containing digestive enzymes, saliva makes it easier to chew and swallow foods (especially dry foods).

Once food has been converted into a soft, moist mass, it's pushed into the pharynx at the back of the mouth and is swallowed. When we swallow, the soft palate closes off the nasal passages from the throat to prevent food from entering the nose.

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