"Uh, oh," Jerry says, looking into his gym bag. "I forgot my toothbrush again."

Jerry, who is 8 years old, is on his way to his dad's for the weekend. His parents are divorced, so he spends the weekdays at his mom's house and most weekends at his dad's house. He has two bedrooms, two sets of pajamas and two piles of comic books to read. Sometimes, it can get confusing trying to remember which of his stuff is at which house. Like his toothbrush.

"Let's buy you a toothbrush to leave here," Jerry's dad says. "That way you won't have to try to remember it. But you do have to remember to brush your teeth."

"I know," Jerry groans. "But I wish I didn't have to. Can't I just skip it on the weekends?" (Jerry has always thought tooth brushing is kind of boring.)

"No way," his dad says. "Anyway, brushing is a lot better than getting cavities. You've got a lot of your permanent teeth now, and you need to take care of them."

Cavities are little holes that can develop in the hard enamel surface of teeth. If the little holes form, germs can get inside your tooth and start eating away the softer material called dentin below the enamel. A cavity is kind of like an infection in your tooth. When dentists put "fillings" in people's teeth, they drill away the infected tooth material and replace it with metal or plastic.

Cavities begin when bacteria that live in your mouth change the food you eat into a powerful acid. The bacteria stick to your teeth in the form of a thin, colorless sticky film called plaque. When food meets plaque, the result is acid. The acid is so strong that it can actually eat holes in your teeth -- unless you don't give it a chance. When you brush, you get rid of the bits of food and the plaque and of any acid that has already formed in your mouth after a meal. But plaque starts reforming on your teeth right away, though. That's why you really should brush after every meal.

Everyone has plaque -- even babies! For them, protecting teeth from plaque begins even before their teeth have broken through their gums. Pediatricians tell parents to wipe their baby's gums with a damp washcloth or cloth pad after each feeding. And they should start brushing their baby's teeth as soon as the teeth appear -- about six months after birth. It's important to care for baby teeth because permanent teeth are just below them in the jaw. If baby teeth get cavities, permanent teeth can be harmed, too.

Jerry's dad bought a brand-new, kid-size toothbrush with a red and white striped handle for his son. That night, Jerry and his dad brushed their teeth at the same time. His dad showed him how to do it right:

1. Use a soft-bristled brush.

2. Squeeze a pea-sized blob of toothpaste onto the brush.

3. Hold the brush bristles at an angle against your teeth. Move the brush back and forth with short, gentle strokes. Brush the inner, outer and top surfaces of your teeth. Don't forget the inside surfaces of your front teeth. Tilt the brush and make several short, up-and-down strokes to do this.

4. Gently brush the inside of your cheeks and the top of your tongue.

5. Rinse with water.

"Good night, Dad," Jerry says.

"Not so fast. Time to floss."

Jerry and his dad both carefully slide dental floss between their teeth until it reaches the gum lines. (This is a job to do gently so you don't hurt your gums.) Finally, Jerry and his dad admire their teeth in the bathroom mirror before they go off to bed.

It's especially important for you to brush your teeth before you go to bed at night, like Jerry did. Otherwise, the plaque on your teeth and the food you had for dinner will have all night to create cavity-forming acid. So do a really good job.

Rinsing with a fluoride rinse is a good idea, too. Fluoride is a substance that makes the shiny enamel coating on your teeth stronger. If you make a habit of brushing your teeth, eat well and see your dentist regularly, you have a pretty good chance of avoiding cavities altogether.

Tips for Parents

These tooth-care tips come from Jed Best, a pediatric dentist and consultant to Kinder-Care Learning Centers Inc., the largest provider of child-care in the United States. Best recommends that by age 6, kids should be using an over-the-counter fluoride mouthwash daily. By age 8, they should be able to floss alone or under your supervision. Brushing should be done with a child-size brush with soft, end-rounded bristles. These are less likely to injure gum tissue. Replace the brush about every three or four months. Should you go electric? Best says that studies have not shown electric toothbrushes to be superior in removing plaque. Whichever brush your child is most likely to use -- manual or electric -- is the best choice.

Catherine O'Neill is a children's writer.