Teeth

Teeth are the hard white parts in your mouth that bite and chew food. They sit in two rows, one on the top jaw and one on the bottom. Humans use teeth to break food into small pieces so the rest of the digestive system can finish the job. Teeth also help us speak clearly and shape the look of our smile.

A human tooth has two main parts. The crown is the part you can see above the gums. The root is the part hidden inside the jawbone, holding the tooth in place. Inside every tooth are three layers. The outside layer is enamel, which is harder than bone. Under the enamel is dentin, a softer layer that gives the tooth its shape. In the very center is the pulp, full of nerves and blood vessels. The nerves are why a cavity hurts.

People grow two sets of teeth in a lifetime. The first set is called baby teeth or milk teeth. Babies usually get their first tooth around six months old, and they have all 20 baby teeth by age three. Starting around age six, those teeth begin to fall out one by one. Adult teeth push up from underneath. A full adult mouth holds 32 teeth, including four wisdom teeth that come in during the late teens or early twenties. Some people never grow wisdom teeth at all, and scientists think human jaws are slowly getting smaller over many generations.

Different teeth do different jobs. The flat front teeth, called incisors, slice food like scissors. The pointed teeth next to them, called canines, grip and tear. The wider teeth in the back, called molars, grind food into mush. Run your tongue across your teeth and you can feel all three shapes.

Teeth can last a whole lifetime, but only with care. Sugary food feeds bacteria that live in your mouth. Those bacteria make acid, and the acid eats holes in enamel called cavities. Once enamel is gone, your body cannot grow it back. Brushing twice a day, flossing, and skipping too many sweets keep teeth strong.

Other animals have very different teeth. Sharks grow new teeth their whole lives and may go through 30,000 in a lifetime. Elephants replace their giant molars six times. Snails have thousands of tiny teeth on their tongues. Humans only get one second chance, so the adult set has to last.