Has this happened to you? You push your tongue up against one of your teeth, wiggle it gently -- and it MOVES. It's a strange feeling to have that old familiar tooth start to wobble around in your mouth -- but it's completely normal.

All human beings get two complete sets of teeth. The first set fits into a child-sized jaw. These are the primary teeth, and many people call these the baby teeth. As your jaw grows to its adult size, another complete set of teeth moves in. These permanent teeth are bullies -- they push the baby teeth right out of your mouth.

When you were born, you didn't have any teeth showing through your gums at all. But they already had started to grow in your jaw. In fact, your teeth began to develop even before you were born. But they didn't begin to appear in your mouth until you were about 6 months old.

As you grew, your baby teeth slowly began to grow through the gums. This is called teething. If there's a baby in your house, you know that teething can be unpleasant, because it makes the gums sore and tender.

Once all of your primary teeth have appeared, you have a set of 20; 10 on the top, 10 on the bottom.

They come in several different shapes. Take a look at them in the mirror. Your front teeth, designed for cutting, are called incisors. On either side of the incisors are your pointed canine teeth. The word canine means "dog-like." Look at your pointed canines, and compare them to a dog's teeth. Can you see the resemblance?

Two-pointed teeth called bicuspids come next. These teeth can tear and grind food. At the back of your mouth are the hard-working molars -- broad, flat teeth that grind. They're the largest teeth in your mouth. You have eight molars in your first set of teeth. When all your permanent teeth come in, you'll have 12.

It's important to take care of your baby teeth. A healthy mouthful of primary teeth provides a good place for permanent teeth to grow and helps them grow straight. If your primary teeth are full of cavities, the decay can spread to the permanent teeth.

The first permanent teeth to appear in your mouth are a set of four molars. Because most kids are about 6 when this happens, dentists call the teeth the "six-year molars." There's nothing to worry about if you're 6 and the molars haven't appeared yet. As your dentist can probably show you on an X-ray, they're up in your jaw getting ready to move down into your mouth.

As you begin to shed your baby teeth, those six-year molars will come in handy. You can use them to chomp on your food when some of your other teeth are loose or missing.

The six-year molars also help shape the lower part of your face. Once all four of these new permanent teeth are in place, they hold your jaws in position while your baby teeth gradually fall out and are replaced by larger permanent teeth. No wonder dentists often say that the six-year molars are the most important teeth in your mouth.

Dentists call the small, white baby teeth your deciduous teeth. That's a fancy word that means the teeth aren't permanent. They eventually fall out. Have you ever heard your science teacher talk about deciduous trees? Those are trees with leaves that fall off in the autumn. The word deciduous means "falling."

You probably already know that some of your baby teeth don't just fall out the way a leaf drifts from a tree, however. They may be loose and bothersome for a few weeks before they're ready to come out.

Have you ever seen cartoons in which the characters pull a loose tooth by tying a string from the tooth to a door knob and then slamming the door? Don't try it! You shouldn't try to force a tooth out until it's almost ready to fall out on its own.

It's okay to push at your loose tooth with your tongue. If you push it too much, however, the gums around the tooth may get red and sore. It's fun to be able to wiggle a loose tooth back and forth -- but until it's ready to fall out, this won't do much more than irritate your gums.

Each of your teeth has a root that anchors it in your jaw. As your permanent teeth force your baby teeth out, these roots slowly disappear. By the time your baby teeth actually fall out, they may not have any root left at all. Then, the loose tooth will come out easily. Just a light tug, or even a slight push with your tongue, may do the trick.

When enough of the root has disappeared, your loose tooth should just lift right out. You may have to give it a little tug, but it won't hurt for more than a second or so. If your gum bleeds a little, put gauze or cotton on it until it stops. Then smile! That gap-toothed grin is a sure sign that you're growing up. Tips for Parents

"Shedding teeth is a natural phenomenon," says Dr. Charles L. Broring, chairman of the department of pedodontics at Georgetown University's School of Dentistry. "Some kids are very defensive about the first one, but they get used to it."

If your child needs help getting a loose tooth out, you can grasp it with a sterile gauze pad, Broring says. "Just pinch the pad over the tooth and wedge it off."

Don't try this method if the tooth can't be flexed back and forth easily, however. The root may still be too long. Extracting primary teeth before they're ready to come out can cause problems with the spacing of permanent teeth.

In the unlikely event that your child gets upset by the loss of baby teeth, have your dentist show him or her the X-rays of the permanent teeth "waiting in the wings."

"That makes those invisible teeth a reality," says Broring.

As for the tooth fairy, Broring says, "The tooth fairy does us dentists a favor, because children look forward to losing their teeth."