Jenny's older brother spends a lot of time looking in the mirror these days. He's a teenager, and he's always worried about how he looks. He puts gel on his hair to spike it up, and he tries lots of different creams and medicines to make his skin clear up. Like most teenagers, Jenny's brother has some acne on his face.

There are lots of slang words for those bumps that appear on the skin. When she's mad at her brother, Jenny sometimes wants to tease him and call him "pizza-face." But she doesn't. After all, he's a lot bigger than she is. And he's not all that bad, as brothers go.

No matter what you call it, acne isn't much fun. About the only good thing to say about pimples is that just about everybody else gets them, too.

Jenny's beginning to worry that her skin is going to start to break out. She's almost 12, and she can tell that her body is beginning to change. Does that mean her skin will, too? Probably. According to the National Center for Health Statistics, only about one quarter of Americans between the age of 12 and 17 have perfectly clear skin. During those years, oil-producing glands on the face, chest, back and shoulders become more active in both boys and girls. The glands get busy under the influence of the hormones that cause puberty -- the process that changes children into adults.

Active oil-producing glands often equal acne. There are thousands of oil glands in your skin -- as many as 20,000 on your face alone. Each gland has a tiny duct, or opening, to the surface of your skin. As the glands make oil, the cells lining the ducts may get plugged up. But the glands keep on churning out oil, which can't get through the plugged-up ducts and escape onto the surface of your skin.

Before long, a duct will get so clogged with oil that it ruptures, or breaks open. The oil spreads into the surrounding skin. The oil is irritating and causes the skin to swell. As if that weren't enough, the body produces a thick, white material called pus as it tries to reabsorb the oil. The result: an angry-looking, swollen pimple.

Luckily, hormone production calms down as you get older, so acne usually clears up in adulthood. (But not always; some adults continue to have acne, which makes them pretty mad. "I thought this was a teenage problem," they'll complain.)

You're likely to get acne if other members of your family have had it. Like hair color or the shape of your nose, a tendency to develop acne as a teenager is part of your heredity.

There's not a whole lot you can do about your heredity. And you can't stop the hormones that your body produces during puberty and adolescence. But there are a few things you can do to keep the skin problem under control. These suggestions come from the American Academy of Pediatrics, an organization of doctors who specialize in taking care of kids and adolescents.

If your skin is broken out, here are some things to avoid: Don't squeeze or pop your pimples. This forces oil from the pores into the skin surrounding them. Then the normal skin swells up, too.

Don't scrub your face hard. That irritates it.

Try to keep hair, hair bands and chin straps off your face. They cause irritation where they rub against the skin.

Don't use oily cosmetics or heavy moisturizers, which can block oil ducts and make acne worse.

Get enough sleep. The body makes extra hormones when it's tired. And extra hormones means extra oil.

If your skin breaks out, people may tell you it happened because your face was dirty, or you ate too much chocolate. But those are myths. And doctors have found no evidence that soft drinks, greasy food or chocolate cause acne. (Oddly enough, working in a fast-food store, which many teenagers do, can make acne worse. That's because the grease in the air gets on the face and adds to its oiliness.)

Acne is one of those things that teenagers have to learn to live with. It's not going to kill you, but it can be pretty upsetting to have a bad case. Luckily, severe acne can be treated and controlled with medicine.

If you have bad acne, ask your parents to talk to a dermatologist. There are things you can do while you wait for your hormones to calm down. Tips for Parents

Acne can be treated with both internal and external medication. Topical benzoyl peroxide in a 5 or 10 percent solution lotion or gel is the most effective treatment available without a prescription. But follow the directions carefully, dermatologists advise. Other external preparations include Retin-A and external antibiotics. Internal medicines include Accutane and certain antibiotics. Accutane is a strong chemical reserved for those people who have severe cystic acne that does not respond to other treatment. It should only be used within strict guidelines under the close supervision of a physician. Using Retin-A can make the skin more sensitive to the sun and can cause redness and peeling; it should be used cautiously. For more information, ask your physician about an American Academy of Pediatrics booklet "Guidelines for Teens: Acne Treatment and Control."

Catherine O'Neill is a freelance children's writer.