Human teeth are complex structures. The hard material of the teeth are composed of calcium, phosphorus, and other mineral salts. The material in the majority of the teeth is called dentine. The hard, shiny layer that people brush is called the enamel. The first set of teeth consists of twenty teeth: there are ten teeth in each jaw. The primary teeth begin to appear when an infant reaches the age of six months. They continue to emerge until the child is approximately two years old. The first permanent tooth usually erupts at the age of six, and a mixture of primary and permanent teeth coexist until the child reaches about twelve years old. By the age of twelve, most children possess 28 of the 32 permanent teeth. The average timetable varies among individuals, but this time variation is not important. The sequence of when each kind of tooth emerges is more important than the age at which this occurs. In fact, deviations in the order of sequence of types of teeth often indicate the possibility of future
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