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Teeth are fascinating for all sorts of reasons. They are the hardest material (enamel) in our body and are absolutely necessary for processing (chewing) food. Because they are made of enamel, teeth last a long time, many becoming fossils when they are buried, and then they can be used to trace evolution in mammals and other animal groups. Teeth show lots of variation; that is, there are different kinds of teeth that animals use for specialized feeding. All of these teeth fit into four categories: incisors (front biting teeth), canines (sharp stabbing teeth), and premolars (or bicuspids) and molars (grinding teeth). Most mammals have only two sets of teeth during their lives. This is called diphyodonty. Because mammals' principal way of feeding when they are young is by nursing on their mother's milk, they do not need teeth immediately after they are born. Sometime after birth, the deciduous or milk teeth begin to come in. In humans, these milk teeth begin to come in through the gums or erupt between ages five and eight months. At this age, children begin to be fed soft food that they do not need to chew. All of the 20 milk teeth are in place by 2 ½ or 3 years, and by this time children are no longer nursing but rather eating normal food that requires chewing. Yet by ages 5 or 6 years, a child's jaws have been growing and thus she or he needs bigger and stronger teeth. This is why children at these ages have teeth that look too small and somewhat like picket fences. At this time, their permanent teeth are already in the gums and begin coming in and pushing out the milk teeth. Most of these permanent teeth are erupted and in place during the teenage years. There will eventually be 32 teeth if all of them come in, although often our last molars (wisdom teeth) do not erupt.

There are some differences among mammalian species in this pattern of diphyodonty, such as in some whales, rodents, elephants, and kangaroos. The two most primitive mammals, the platypus and echidna, have no teeth at all. But all of these mammals still nurse their young.

However, the general pattern for most mammals, including humans, is one where milk teeth erupt some time after birth when nursing does not require a full set of teeth for very young animals to feed properly.

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