Babies are born with 20 baby teeth already hidden in their jaws. You grow two sets of teeth in your life, baby teeth first and then adult teeth. Baby teeth fall out during childhood. The adult teeth that come next have to last for the rest of your life.

A baby's first tooth breaks through the gum around 6 months old, and it is most often one of the two bottom front teeth. The four upper front teeth tend to follow about one to two months later.

Teeth help us bite and chew our food. The sharp front teeth cut food when we bite, and the molars in back are for grinding and chewing. Chewing breaks food into a soft ball that can be swallowed. It is the first step of digestion. Teeth help you talk, too. Your tongue taps against your teeth or the roof of your mouth to make some sounds. Your teeth also help shape words by controlling how air flows out of your mouth.

Enamel is the hard outer coat of a tooth, and it is the hardest part of the human body. 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, and that is why protecting it matters. Under the enamel is dentin. Dentin is made of living cells and 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.

A tooth has a crown above the gum and a root below it. The soft pulp in the middle of a tooth holds its nerves and blood vessels. Cementum covers the root. When gums pull back from the teeth, the soft root surface is exposed. Cementum is softer than enamel, so roots decay more easily and are more sensitive to hot and cold.

Plaque is a sticky film of bacteria that keeps forming on your teeth. It 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. When you eat or drink sugary foods, the bacteria make acid. Plaque you do not clean off each day can harden into tartar, a rough substance that builds up along the gum line. Once tartar has formed, only a dentist can remove it.

Sipping and snacking all day keeps teeth under acid attack. After sugar, mouth bacteria make acid, and it takes time for saliva to neutralize it. If teeth are exposed to acid again and again, the enamel keeps losing minerals and can break down into a cavity. Cavity bacteria can use any sugar, even in foods that seem healthy. Fruit juice, honey, and cereal bars made with honey or molasses all contain sugar. Tooth decay is caused by too much sugary food and drink and by not cleaning your teeth and gums regularly.

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. Early tooth decay usually does not hurt, so a dentist check is how it gets found. Dentists can see decay that your eyes cannot. They check for soft or sticky spots and take X-rays.

Many towns add fluoride to tap water because it helps teeth resist cavities. Scientists studied water fluoridation for decades. Trendley Dean set out in 1931 to study fluoride's harm, and 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 protects teeth at every age, in children and in adults. Fluoride toothpaste and mouth rinse are two ways to get fluoride onto your teeth.

Gum disease is an infection that inflames the tissues holding your teeth in place, and it starts with swollen, red and bleeding gums. Use a soft-bristled toothbrush. The bristles of a stiff brush can wear away the thin cementum that covers the root. Clean between your teeth every day with floss or a small brush made for the job. Brush your teeth two times a day, and use a toothpaste with fluoride when you brush.

Words to know

• plaque: sticky film
• enamel: hard tooth coat
• dentin: living layer inside
• cementum: root cover
• fluoride: cavity fighter