Teeth help us bite and chew our food, and chewing breaks the food into a soft ball that can be swallowed, which is the first step of digestion. People have four kinds of teeth, called incisors, canines, premolars and molars, and the sharp front incisors cut food when we bite while the molars in back are for grinding and chewing. Teeth also help us talk, because the tongue taps against the teeth or the roof of the mouth to make some sounds, and the teeth help shape words by controlling how air flows out of the mouth. They help shape our faces as well.
People grow two sets of teeth in a lifetime, which makes humans diphyodont. Babies are born with 20 baby teeth already hidden in their jaws, and those baby teeth fall out during childhood and are replaced by an adult set of 32 permanent teeth, which has to last for the rest of a person's life. Other animals handle replacement very differently, because the teeth of reptiles are replaced constantly throughout their lives and most vertebrates replace their teeth over and over, so people are an exception because we get only two sets.
Baby teeth have roots, and the adult front teeth form behind them, so something has to clear the way as the adult tooth comes in. Cells called odontoclasts break down the root of a baby tooth to make room for the adult tooth, dissolving the root with acids and enzymes and leaving small pits along the root surface. The process speeds up and slows down in phases, and the order in which teeth come in matters more than the exact timing, so a shift of up to 6 months from the usual date is still normal.
Enamel is the hard outer coat of a tooth, and it is the hardest part of the human body, harder even than bone. It is mostly hydroxyapatite, a hard crystal built from calcium and phosphate, and it is thinnest near the gum line and thickest on the biting surfaces and cusps, where the most chewing force lands. Cells called ameloblasts make enamel, but they work only before the tooth comes in, and because mature enamel is not alive, it has no living cells and cannot feel or repair itself. Once enamel is formed it cannot grow back, which is why protecting it matters. 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, but a 2024 study found that it does not give the teeth their orange color, as people long believed.
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-hued, bone-like material that greatly affects the color of a tooth. Because it is slightly softer than enamel and a little elastic, it flexes and absorbs force and keeps the hard but brittle enamel from cracking. Dentin wears away faster than enamel because it is less mineralized, but unlike enamel it keeps forming throughout life, with secondary dentin forming through the years and reparative dentin forming when the tooth is irritated or injured.
Dentin is older than teeth. In early vertebrates it was an important part of the bony skin armor that covered most of the body, and today it persists in a few living animals such as the coelacanth. In human teeth it lies under the enamel and surrounds the pulp, the soft center that holds the tooth's nerves and blood vessels. Because dentin is full of microscopic tubes called dentinal tubules, which hold fluid and the thread-like extension of a dentin-making cell and connect with the nerve in the pulp, heat, cold and decay can cause pain, and that is why dentin hurts when something is wrong.
Three different cell types build the three hard tissues of a tooth, since ameloblasts make enamel, odontoblasts make dentin and cementoblasts make cementum, which covers the root. Cementum is bone-like and softer than enamel, and a web of fibers called the periodontal ligament attaches it to the jawbone, so a tooth is not stuck in the jaw like a nail. The ligament helps the tooth withstand the large forces of chewing. Tooth anatomy varies widely across animals, and in whales the cementum covers the whole tooth surface with only a small cap of enamel at the tip.
Plaque is a living community of bacteria, not just leftover food. About 70 percent of its dry weight is bacteria, and the rest is sugars and proteins. Within hours, bacteria begin to bind to the thin film on the tooth, and one of them, Streptococcus mutans, builds its own glue by turning sucrose into dextran, a sticky polysaccharide that helps it stick to the tooth surface and makes it the leading cause of tooth decay. Plaque that you do not clean off each day can harden into tartar, a rough substance that builds up along the gum line, and once tartar has formed, only a dentist can remove it.
Whether a tooth loses minerals depends on acidity. When you eat or drink sugary foods, bacteria in plaque make acid, and 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, and if teeth are exposed again and again, the enamel keeps losing minerals and can break down into a cavity, even when the sugar comes from foods that seem healthy, such as fruit juice or honey.
Enamel, dentin and cementum are always swapping minerals with the mouth in a back and forth process called demineralization and remineralization, and before a cavity forms, this process can be reversed. The first sign of a cavity starting can be a white spot on a tooth, where acid has pulled minerals out of the enamel. At this early stage the damage can be reversed, because minerals from saliva and fluoride can go back into the enamel, so it may be possible to stop early decay with fluoride. Later, if the process is not stopped, a hole, or cavity, can form.
Fluoride works by strengthening the hard outer surface of a tooth, the enamel, so that it resists decay better, and it reaches the teeth of children and adults alike from toothpaste, from a mouth rinse and from tap water in many towns. Almost all water has some natural fluoride, but usually too little to prevent cavities, and the amount depends on the water source. Although many towns add fluoride, the CDC does not require any town to add it, because state and local governments decide whether their water is fluoridated.
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 link that a UK dentist, Norman Ainsworth, had reported in 1925. Trendley Dean set out in 1931 to study fluoride's harm, but by 1950 he had shown that small amounts prevent cavities, and today more than 100 health organizations recognize the cavity-fighting benefits of fluoridated water.
Sharks show how different replacement can be. A shark's teeth sit in rows, and when a front tooth falls out, a fresh one from the rows behind moves forward like a conveyor belt, so that some sharks shed about 35,000 teeth during their lives. Shark teeth are often the only part of a shark to be fossilized, and the oldest shark-like fish, which date back about 450 million years to the Late Ordovician period, are known mostly from fossil teeth and skin scales, which means that fossil teeth make up much of the record for the whole shark group.
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. The scientist who studied the mole rat thinks the teeth help it dig rather than eat, because it digs hard, dry soil with its front teeth, which grinds its molars, and it swallows abrasive dust. This is a hypothesis, not yet proven, and mammals in general traded an unlimited supply of teeth for more complex shapes, which lets them eat a wider range of foods. Elephants wear their teeth in a different way. An elephant can go through up to six sets of molars in its lifetime, because its molars move forward in the jaw as they wear down and a worn-down tooth is lost when it reaches the front.
Wisdom teeth are the last molars, at the very back, and they come in later than the other teeth. Early ancestors ate tough, uncooked foods that wore teeth down, so their big back molars were helpful, but as cooking and softer foods spread, jaws got smaller, and now wisdom teeth often have no room to come in. A wisdom tooth in the wrong position can trap food and give cavity-causing bacteria a place to grow. 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.
Tooth decay can happen at any age, from childhood to old age. As of November 2026, cavities are among the most common childhood health problems, and in the US about half of children aged 6 to 9 have had at least one, while nearly all adults over 65 have had a cavity. As of March 2025, the World Health Organization estimates that oral diseases affect nearly 3.7 billion people, and untreated tooth decay in permanent teeth is the most common health condition measured by the Global Burden of Disease study (2021).
Words to know
- hydroxyapatite: hard tooth crystal
- ameloblasts: enamel-making cells
- odontoclasts: root-dissolving cells
- cementum: root covering
- diphyodont: two-set teeth
