Babies are born with 20 baby teeth. The teeth are 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. 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. Most children have all 20 baby teeth by about age 3.

Teeth help us bite and chew our food. The sharp front teeth cut food when we bite. 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. Your tongue taps against your teeth or the roof of your mouth to make some sounds. Your teeth help shape words by controlling how air flows out of your mouth. Teeth also help shape our faces. Baby teeth help children speak clearly.

Each tooth has four parts: enamel, dentin, cementum and pulp. Under the hard coat is dentin. It is made of living cells. It makes up the biggest part of a tooth. The soft pulp in the middle holds its nerves and blood vessels.

Plaque is a sticky film of bacteria. It keeps forming on your teeth. When you eat or drink sugary foods, the bacteria make acid. The acid attacks the enamel. Snacks and drinks with a lot of sugar are bad for teeth, so it is smart to limit them.

A cavity can start with a white spot on a tooth. This is 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 in. Early tooth decay usually does not hurt, so a dentist check is how it gets found.

Spit does more than keep your mouth wet. It moistens and breaks down food. It washes food bits off your teeth. It also carries minerals such as calcium and phosphate. These help keep teeth strong and fight tooth decay. Tooth decay can happen at any age, from childhood to old age.

A shark keeps growing new teeth all its life. Its teeth sit in rows. The front ones fall away, and fresh rows move forward like a conveyor belt. Some sharks shed about 35,000 teeth during their lives. When a front tooth falls out, a fresh one from the rows behind moves forward to take its place.

Enamel is the hard outer coat of a tooth. It is the hardest part of the human body. It is harder even than bone. Use a toothpaste with fluoride when you brush. Enamel cannot grow back once it is formed, so protecting it matters.

Words to know

• enamel: hard tooth coat
• dentin: layer under enamel
• plaque: sticky film
• fluoride: cavity fighter

The short version

A tooth has four parts: enamel, dentin, cementum and pulp. Enamel is the hard outer coat, and it cannot grow back once it is formed. People grow two sets of teeth, 20 baby teeth first and then 32 adult teeth that must last a lifetime.

Quick facts

• Enamel is the hardest part of the human body.
• Babies are born with 20 baby teeth already hidden in their jaws.
• Adults have 32 permanent teeth.
• Brush two times a day with a fluoride toothpaste.
• Sealants cut decay in the back molars by nearly 80 percent.