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. 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. 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 help shape words by controlling how air flows out of your mouth.

Each tooth has four parts, and they are enamel, dentin, cementum and pulp. Enamel is the hard outer coat of a tooth, and it is the hardest part of the human body. It is harder even than bone. Mature enamel is not alive, and 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. It is made of living cells and makes up the biggest part of a tooth. Dentin is alive. It can sense hot and cold. It is the part of a tooth that hurts when something is wrong. The soft pulp in the middle of a tooth holds its nerves and blood vessels. Cementum covers the root of a tooth. It is bone-like. It is softer than enamel.

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. Cavity bacteria turn sugar into acid, even when the sugar is in foods that seem healthy. Fruit juice has sugar the bacteria can use. So does honey. So do cereal bars made with honey or molasses. It is not just candy.

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, which is why cleaning every day matters. Tartar can lead to gum disease, and gum disease is an infection that inflames the tissues holding your teeth in place. It starts with swollen, red and bleeding gums.

Tooth decay can happen at any age, from childhood to old age. It is not only a problem for children. When gums pull back from the teeth, the soft root surface is exposed. Roots are covered by cementum, which is softer than enamel, so they decay more easily. They are also more sensitive to hot and cold. Spit helps. It washes food bits off your teeth and carries minerals that help keep teeth strong.

Early tooth decay usually does not hurt, so a dentist check is how it gets found. Dentists can see decay that your eyes cannot see, even when a tooth looks fine to you. They check each tooth for soft or sticky spots, and they also take X-rays. In a bitewing X-ray, you bite down on a small tab. It is a routine way to find decay that is hiding between teeth.

Many towns add fluoride to tap water, because fluoride helps teeth resist cavities. Scientists studied water fluoridation for decades. Trendley Dean set out in 1931 to study the harm of fluoride, but 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.

Words to know

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