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A tooth is a hard structure in the jaw or mouth of many species that has a variety of functions. Teeth work together in a set to chew, grind or tear up food, with different types of teeth involved in the process of eating. In humans, for example, the sharp incisors are found nearer the front of the mouth, and are responsible for breaking up food, while the molars further back grind the contents of the mouth down prior to swallowing. Teeth are different shapes according to their various purposes and the usual diet of the species.

Species vary widely in the types of teeth, their functions, and how many are grown. Some species, such as sharks, grow new sets of teeth throughout their lives. Humans have two, the first being a set of 'milk teeth' that appear in infancy, within a few months of birth. These gradually fall out during childhood and are replaced by the final set of adult teeth, which numbers 32. This includes four 'wisdom teeth' at the back of the mouth, which often do not develop properly, cause pain and have to be surgically removed.

Teeth can also cause health problems if not properly cleaned, because food particles can easily find their way into the areas between and around teeth, allowing bacteria to build up and cause decay. Regular visits to a medical practitioner known as a dentist are necessary to avoid expensive dental work, painful toothache or gum disease.

Teeth are not only useful for chewing food. Many animals use them for fighting with others for mates or territory, or to display their health and strength during their search for mates. A purpose unique to humans is the involvement of teeth in producing spoken language: for example, the sounds [θ] and [ð], found in the English words tooth and teethe respectively, both involve the tongue striking the back or edges of the upper front teeth. In many languages, the sounds [t], [d] and [n] involve some dental contact too.

Scientists make good use of teeth in investigating how species evolved, lived and died. Examination of a tooth can reveal a great deal of evidence about a person or creature's diet, age, health and habits. Teeth can survive fossilization, so provide a wealth of evidence about how species evolved. The fossil record indicates that teeth evolved from denticles (scales) found in early fish.

Teeth play an important role in human culture; for example, the emergence of adult teeth may be significant in a child's life, and can even be rewarded. The 'tooth fairy' in Western society is said to leave money in exchange for milk teeth (though many children suspect that their caregivers may be involved). Elsewhere, teeth may be thrown away as part of a ritual. In criminal inquiries, investigators can make use of tooth marks, dental records and teeth themselves to identify victims, criminals and the circumstances of a crime.