Teeth are a specialized bony tissue found in the upper and lower jaw bones that are used for cutting and grinding food to aid digestion. Like bones, they are primarily formed from calcium. Unlike bones, they contain a soft pulpy tissue inside, contain nerves, and are surrounded by a very hard nearly impenetrable substance called dentine.
Dental caries, or cavities, are the most commonly suffered disease in the world. Almost all humans suffer from dental caries. An untreated dental carie will usually result in an abcess in the underlying tissue. Dental abscesses are among the most painful disorders known and generally require immediate treatment, at least with painkillers, but usually with removal of the tooth or the removal of the living nerves from the tooth (a "root canal").
Humans grow a set of 20 teeth very shortly after birth, all of which are lost during childhood to be replaced by 20 permanent teeth. In addition, 12 teeth, the molars, grow as the jaw gets larger, generally with the first set coming in at the age of six, the second set at age 12, and the third or wisdom teeth coming in at age 18. Most people's wisdom teeth do not come in properly and have to be removed.