Babies are born with 20 baby teeth already hidden in their jaws, and the first of them usually breaks through the gum around 6 months old, most often as one of the two bottom front teeth. The four upper front teeth tend to follow about one to two months later, and most children have all 20 baby teeth by about age 3. Although many parents expect a fever when baby teeth break through, extensive studies show that a fever is a false symptom of teething, so a baby with a fever should be checked for another cause. Baby teeth also do real work, because they help children speak clearly.
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. Children have 20 baby teeth, while 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, dissolving it with acids and enzymes that leave small pits along the root surface. Although 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.
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 but 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. Mature enamel is not alive, so it has no living cells and cannot feel or repair itself. Once it is formed it cannot grow back, which is why 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-hued, bone-like material that greatly affects the color of a tooth, and because it is slightly softer than enamel and a little elastic, it flexes and absorbs force while the hard but brittle enamel above it stays uncracked. Dentin wears away faster than enamel because it is less mineralized, but unlike enamel it can build itself back up, since secondary dentin keeps forming through life and reparative dentin forms when the tooth is irritated or injured.
The soft pulp in the middle of a tooth holds its nerves and blood vessels, and the dentin around it 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. This connection is how heat, cold and decay cause pain, which is why dentin is the part of a tooth that hurts when something is wrong. Mature enamel has no living cells and cannot feel. Early tooth decay usually does not hurt, so a dentist check is how it gets found.
Cementum covers the root of a tooth, so the crown is covered by enamel while the root is covered by a bone-like layer that is 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 to the jawbone and helps the tooth withstand the large forces of chewing. When gums pull back from the teeth, the soft root surface is exposed, and because cementum is softer than enamel, roots decay more easily and are more sensitive to hot and cold. A soft-bristled toothbrush is the better choice, because the bristles of a stiff brush can wear away the thin cementum that covers the root.
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, 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.
When you eat or drink sugary foods, bacteria in plaque make acid, and the acid attacks the enamel. Below a critical pH of about 5.5, the mineral in enamel dissolves. After a glucose rinse, the pH of plaque drops suddenly and 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. Sugar is not only in candy. Fruit juice, honey, and cereal bars made with honey or molasses all contain sugar the bacteria can use.
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. 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 keep teeth strong.
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 both children and adults 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 to their tap water, 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 of it prevent cavities, and today more than 100 health organizations recognize the cavity-fighting benefits of fluoridated water.
Sealants work like raincoats for the chewing surfaces of back teeth. Because molars are rough and uneven, food and bacteria hide in them, and a sealant keeps them out, which is why sealants have been shown to cut the risk of decay in molars by nearly 80 percent. Your first adult molars come in around age 6, behind your baby teeth, and your second molars come in around age 12, so dentists like to seal these new back teeth as soon as they come in, before decay has a chance to start. Baby teeth can get cavities too, and that matters, because baby teeth lost to cavities can threaten the health of the permanent teeth that come next.
Dentists can see decay that your eyes cannot, because they check for soft or sticky spots and take X-rays, and a bitewing X-ray, where you bite down on a small tab, is a routine way to find decay hiding between teeth. Since early decay usually does not hurt, these checks matter at every age. Cavities are common in children, and as of November 2026 nearly all adults over 65 have had a cavity. Hundreds of medicines can also cause the salivary glands to make less saliva, which leaves the mouth with less protection and a higher risk of tooth decay.
Sharks can go through a huge number of teeth. Their 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 some sharks shed about 35,000 teeth during their lives. Shark teeth are often the only part of a shark to be fossilized. 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. Fossil teeth make up much of the record for the whole shark group. Georgia chose the shark tooth as its official state fossil in 1976 because shark teeth are found there so often.
Most vertebrates replace their teeth over and over during life, but people are an exception because we get only two sets.
As of November 2026, researchers are trying to regrow the living pulp inside damaged teeth using stem cells. In a small trial of 40 children with injured adult teeth, 30 got stem cells taken from their own baby teeth, and blood flow rose in the treated teeth. The trial authors call their result a first step, and growing whole teeth is not yet possible. In a Tufts University study, damaged human tooth roots were filled with a gel carrying dental stem cells and implanted in rodents, and within weeks new cells and blood vessels grew. As of November 2026, researchers say this supports a possible future route to restoring natural tooth function.
Words to know
- hydroxyapatite: hard tooth crystal
- odontoclasts: root-dissolving cells
- cementum: root covering
- tubules: tiny tubes
- plaque: sticky germ film
