Mammals are diphyodont, meaning that they develop two sets of teeth. In humans, the first set (the "baby," "milk," "primary" or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months is known as teething and can be painful.
Some animals develop only one set of teeth (monophyodont) while others develop many sets (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks to replace worn teeth. Rodent incisors grow and wear away continually through gnawing, maintaining relatively constant length. The industry of the beaver is due in part to this qualification. Many rodents such as voles (but not mice) and guinea pigs, as well as rabbits, have continuously growing molars in addition to incisors.
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