The part of a tooth you can see above the gum is the crown, and the root sits below it. The crown is covered by a coat of enamel, which is the hardest part of the human body. Enamel is even harder than bone. It is mostly hydroxyapatite, a hard crystal built from calcium and phosphate. It is not the same thickness everywhere. Enamel is thinnest near the gum line and thickest on the biting surfaces and cusps, where the most chewing force lands.

People grow two sets of teeth in a lifetime, and the adult teeth that come in after the baby teeth have to last for the rest of a person's life. Enamel is made by cells called ameloblasts, and they do their work only before the tooth comes in. Mature enamel is not alive. It has no living cells, so it cannot feel or repair itself. Once enamel is formed it cannot grow back, so protecting it matters.

Under the enamel is dentin, which is made of living cells and makes up the biggest part of a tooth. Dentin is a porous, yellow, bone-like material, and it greatly affects the color of a tooth. It is slightly softer than enamel and a little elastic. Enamel is hard but brittle, so the dentin underneath flexes and absorbs force and keeps the enamel from cracking. Because dentin is less mineralized than enamel, it wears away faster.

The soft pulp in the middle of a tooth holds its nerves and blood vessels. Dentin is full of microscopic tubes called dentinal tubules. They hold fluid and the thread-like extension of a dentin-making cell, and they connect with the nerve in the pulp. This is how heat, cold and decay cause pain. Dentin is alive and can sense hot and cold, so it is the part of a tooth that hurts when something is wrong. Early decay usually does not hurt, so a dentist check is how it gets found.

Three different cell types build the three hard tissues of a tooth. Ameloblasts make enamel, odontoblasts make dentin and cementoblasts make cementum. Dentin keeps forming throughout life, so it can thicken and repair, but enamel cannot. Secondary dentin keeps forming through life, and reparative dentin forms when the tooth is irritated or injured.

Cementum covers a tooth's root. It is bone-like and softer than enamel. A tooth is not stuck in the jaw like a nail. It hangs in its socket from a web of fibers called the periodontal ligament, which attaches the cementum on the root to the jawbone and helps the tooth withstand the large forces of chewing. Enamel meets cementum at the cementoenamel junction, near the gum line. Use a soft-bristled toothbrush, because the bristles of a stiff brush can wear away the thin cementum that covers the root.

Children have 20 baby teeth, and adults have 32 permanent teeth. Baby teeth have roots, and the adult front teeth form behind them. Cells called odontoclasts break down the root of a baby tooth to make room for the adult tooth coming in. They dissolve the root with acids and enzymes and leave small pits along the root surface. The process speeds up and slows down in phases. The order in which teeth come in matters more than the exact timing, and a shift of up to 6 months from the usual date is still normal.

Rodents such as beavers have teeth that keep growing. Gnawing wears them down quickly, so they stay the right size. A beaver can gnaw underwater because its lips close behind its front teeth, which keeps water out of its mouth. Rodent front teeth also sharpen themselves, because they have enamel on the outside and exposed dentin on the inside. As of November 2026, studies report that rodent enamel is about 96 percent hydroxyapatite by weight and also contains iron. The iron makes the teeth stronger. People long believed it gave the teeth their orange color, but a 2024 study found that it does not.

An elephant can go through up to six sets of molars in its lifetime. Its molars move forward in the jaw as they wear down, and a worn-down tooth is lost when it reaches the front. One molar can weigh about 5 pounds and is the size of a brick. An elephant's tusks are teeth too. They are its front teeth, called incisors, and the only incisors it has. Elephants are born with a first pair of tiny tusks that fall out after about a year, when they are around 2 inches long.

Most vertebrates replace their teeth over and over during life, and people are an exception because we get only two sets. A shark keeps growing new teeth all its life. Its teeth sit in rows, and when a front tooth falls out, a fresh one from the rows behind moves forward like a conveyor belt. Some sharks shed about 35,000 teeth during their lives. The oldest shark-like fish date back about 450 million years, and they are known mostly from fossil teeth and skin scales.

When you eat or drink sugary foods, bacteria in plaque make acid, and the acid attacks the enamel. Early decay starts when acid makes enamel lose minerals, which can leave a white spot. If decay goes on, a hole, or cavity, can form. At the white-spot stage, though, the damage can be reversed, because minerals from saliva and fluoride can go back into the enamel. This repair is called remineralization.

Words to know

• hydroxyapatite: hard tooth crystal
• ameloblasts: enamel-making cells
• dentin: living tooth layer
• brittle: hard but snaps
• plaque: sticky germ film