Babies are born with 20 baby teeth already hidden in their jaws. People grow two sets of teeth in a lifetime, with baby teeth first and then adult teeth. Most vertebrates replace their teeth over and over during life. People are an exception because we get only two sets. Baby teeth fall out during childhood, and the adult teeth that come next have to last for the rest of a person's life. Baby teeth can get cavities too, and that matters, because losing them to cavities can threaten the health of the permanent teeth that come next.

A baby's first tooth usually breaks through the gum around 6 months old, and it is usually one of the two bottom front teeth. The four upper front teeth tend to follow about one to two months later. The order in which teeth come in matters more than the exact timing of each tooth, and a shift of up to 6 months from the usual date is still normal. Your first adult molars come in around age 6, and they come in behind your first teeth. Your second molars come in around age 12.

Children have 20 baby teeth. Adults have 32 permanent teeth in all. Baby teeth do have roots, and adult front teeth form behind them. Cells called odontoclasts dissolve the root of a baby tooth with acids and enzymes. This makes room for the adult tooth that is coming in. They leave small pits along the root of the baby tooth, and the process speeds up and slows down in phases. Baby teeth also help children speak clearly, because your tongue taps against your teeth to make some sounds.

In elephants, the molars move forward in the jaw as they wear down, and a worn-down tooth is lost when it reaches the front of the jaw. One molar can weigh about 5 pounds. It is the size of a brick. An elephant can go through up to six sets of molars in its lifetime. A few other kinds of mammals also keep getting new molars all through their lives. The silvery mole rat of East Africa grows new molars at the back of its jaw, and they slowly move forward while the worn front ones are absorbed.

Enamel is the hard outer coat of a tooth, and it is the hardest part of the human body. It is thinnest near the gum line, and it is thickest on the biting surfaces and cusps, where the most chewing force lands. Enamel is hard but brittle, so the dentin underneath, which is slightly softer and a little elastic, flexes and absorbs force and keeps the enamel from cracking. A mouthguard also cushions a blow to the face and lowers the risk of broken teeth. Upper teeth take most injuries because they stick out farther.

Dentin is made of living cells, and it makes up the biggest part of a tooth. It is alive and can sense hot and cold, and it is the part of a tooth that hurts when something is wrong. 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. The pulp is the soft center of the tooth, and it holds the nerves and blood vessels of the tooth.

Cementum covers the root of a tooth, and it is bone-like and softer than enamel. A membrane called the periodontal membrane attaches to the cementum and bonds the root of the tooth to the jawbone. When gums pull back from the teeth, the soft root surface is exposed. Cementum is softer than enamel. So roots decay more easily, and they are more sensitive to hot and cold. Gum disease is an infection that inflames the tissues holding your teeth in place, and it starts with swollen, red and bleeding gums.

Plaque is a sticky film of bacteria that keeps forming on your teeth. When you eat or drink sugary foods, bacteria in plaque make acid. Sipping and snacking all day keeps teeth under acid attack, because it takes time for saliva to neutralize the acid. Early decay starts when acid makes enamel lose minerals, which can leave a white spot. At that stage decay can be stopped or reversed. Later a hole, or cavity, can form. Decay is caused by too much sugary food and drink and by not cleaning your teeth and gums regularly.

Fluoride works by strengthening the tooth's hard outer surface, the enamel, so that it resists decay better. Fluoride from toothpaste and from tap water in many towns strengthens the enamel surface. It reaches teeth in both children and adults. Almost all water has some natural fluoride. Usually it has too little to prevent cavities. Trendley Dean set out in 1931 to study fluoride's harm, but by 1950 he had shown that small amounts of fluoride prevent cavities. The CDC does not require any town to add fluoride, so state and local governments decide.

Tooth decay can happen at any age, from childhood all the way to old age. As of November 2026, nearly all adults over 65 have had a cavity, so tooth care matters for life. Cells called ameloblasts make enamel, and only before the tooth comes in. Dentin keeps forming all through life, so it can thicken and repair. Enamel cannot. When a tooth is hurt, the dentin forms new layers, called reparative dentin. Mature enamel is not alive, so it cannot feel or repair itself. Once formed, it cannot grow back, so protecting it matters.

Words to know

• odontoclasts: root-dissolving cells
• enamel: hard tooth coat
• dentin: living tooth layer
• cementum: root covering
• plaque: sticky film