Enamel is the hard outer coat of a tooth, and it is the hardest part of the human body, even harder than bone. It is mostly hydroxyapatite, a hard crystal built from calcium and phosphate, and it is made by cells called ameloblasts. They work only before the tooth comes in. Once a tooth is in the mouth, mature enamel is not alive. It has no living cells, so it cannot feel or repair itself, and it cannot grow back after it is formed. That is why protecting it matters.

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

Dentin is alive, and it is full of microscopic tubes called dentinal tubules that hold fluid and the thread-like extension of a dentin-making cell. These tubes connect with the nerve in the pulp, the soft center of the tooth that holds its nerves and blood vessels, and this is how heat, cold and decay cause pain. Dentin also keeps forming throughout life. Secondary dentin keeps forming as the years pass, and reparative dentin forms when the tooth is irritated or injured.

Cementum covers the root of a tooth. 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. The crown is covered by enamel and the root by cementum, and the two meet at the cementoenamel junction, near the gum line.

Plaque is a living community of bacteria, not just leftover food, and about 70 percent of its dry weight is bacteria while the rest is sugars and proteins. Within hours, bacteria begin to bind to the thin film on the tooth. One of them, Streptococcus mutans, builds its own glue to hold on. It turns sucrose into dextran, a sticky polysaccharide that helps it stick to the tooth surface. It is the leading cause of tooth decay.

Whether a tooth loses minerals depends on how acid its surroundings are. Below a critical pH of about 5.5, the mineral in enamel dissolves. After a glucose rinse, the pH of plaque drops suddenly, and it returns to normal after 30 to 60 minutes as saliva buffers the acid. Sipping and snacking all day keeps teeth under acid attack, because it takes time for saliva to neutralize the acid. If teeth are exposed again and again, the enamel keeps losing minerals and can break down into a cavity.

The first sign of a cavity starting can be a white spot on a tooth, where acid has pulled minerals out of the outer coat. At this early stage the damage can be reversed, because minerals from saliva and fluoride can go back into it, and this repair is called remineralization. Saliva does more than keep your mouth wet. It moistens and breaks down food, washes food bits off your teeth, and carries minerals such as calcium and phosphate that help fight tooth decay.

Water fluoridation began with a puzzle. Dentist Frederick McKay spent thirty years investigating the Colorado brown stain, a mottling of enamel, and found that stained teeth were also cavity-free. A UK dentist, Norman Ainsworth, had reported the same link in 1925. Trendley Dean set out in 1931 to study fluoride's harm, but by 1950 he had shown that small amounts prevent cavities. Today more than 100 health organizations recognize the cavity-fighting benefits of fluoridated water.

Fluoride works by strengthening the hard outer surface of the tooth, the enamel, so that it resists decay better. Almost all water has some natural fluoride, but usually too little to prevent cavities, and the amount depends on the water source. Fluoride from toothpaste and from tap water in many towns reaches teeth in both children and adults, but the CDC does not require any town to add it. State and local governments decide.

Most vertebrates replace their teeth over and over during life, and people are an exception because we get only two sets. A few mammals keep getting new molars all their lives. The silvery mole rat of East Africa grows new molars at the back of its jaw that slowly move forward while the worn front ones are absorbed, and manatees and kangaroos also replace their teeth over and over.

Why does the silvery mole rat replace its molars? The scientist who studied it thinks the teeth help it dig rather than eat. It digs hard, dry soil with its front teeth, which grinds its molars, and it swallows abrasive dust along with the dirt. This is a hypothesis, and it has not yet been proven.

Wisdom teeth are third molars that come in late, and trouble comes when the jaw has no room. Early ancestors ate tough, uncooked foods that wore teeth down, so their big back molars were helpful. As cooking and softer foods spread, jaws got smaller, and now wisdom teeth often have no room to come in. In a YouGov survey of 6,308 U.S. adults in December 2024, about two-thirds of people 45 and older reported all their wisdom teeth removed, but only 26% of adults under 30 had had them removed. Part of the gap is that many younger adults have not had enough time for their wisdom teeth to emerge or cause problems.

In 2015 University of Portsmouth researchers reported that limpet teeth may be the strongest natural material known, at up to 6.5 gigapascals of tensile strength, which beats spider silk at 4.5 and Kevlar at 3 to 3.5. Other reports give different numbers for silk, so the comparison is a reported result. Limpet teeth are made of protein frames packed with nanofibers of a mineral called goethite. Most structures get weaker as they get bigger, because bigger parts hold more flaws, but limpet teeth keep their strength at any size.

Words to know

• goethite: strong mineral
• brittle: easily broken
• hydroxyapatite: hard tooth crystal
• plaque: sticky germ film
• ameloblasts: enamel-making cells