Q.I have 3 daughters. The oldest just turned 2 and the twins will be a year old next month. The problem is my 2-year-old.

She's been displaying aggressive behavior for about 2 months: temper tantrums, kicking, throwing toys, screaming, doing the things she knows she's not supposed to do.

I can tolerate these things and keep my cool when correcting her, but the biting has to stop! She's left so many teeth marks the rest of us look abused.

I can't help but pop her one right in the kisser, but I've been told that violence reinforces violence.She doesn't eat all that much, but she's active and very strong. She's also bright, has a good vocabulary and is creative on her own.

A.Misbehavior is like a bellyache. If you treat the symptoms, they may go away, but if you treat the cause, they'll stay away.

The amount your child eats shouldn't be a factor, but she may be reacting to something in her diet, so you should track it to see if there is a connection.

It's more likely that the cause is in her environment -- and it's not one cause but two. With twins, she must feel like she's playing third fiddle instead of second. No wonder she's furious.

A new baby is always hard for an old baby to appreciate, and twins are especially hard, for they take so much more of a mother's time. Jealousy is the curse of the firstborn.

Your firstborn has turned to biting, smiting foolishness, not because she loves too little but too much. You and your husband mean the world to her and she isn't ready to share you.

You surely saw some jealousy when the twins were born and again at about 3 months.

The jealousy became obvious -- and the misbehavior began -- when the twins were 9 months old, the time for sleeping less, moving more and grabbing a big sister's toys. That's when your first child turned from the naturally assertive ways of a toddler into full-fledged aggression.

Although she needs discipline now, she needs attention more. A tantrum is a child's way to say, "Look at me! Notice me!" You want to heed the message, but not at that moment.

Instead, tell her, with sympathy, that she must be tired to be so cross and put her in her crib or leave her in a baby-proofed room where she can't get into too much trouble. In either case, shut the door and go about your business, without answering her or going to her unless you think she -- or the room -- is in danger. Without an audience, she should pull herself together in 10-15 minutes and then you go to her with smiles, hugs and kisses and tell her you're so glad your good little girl is back.

She also learns this lesson when you spontaneously reward her good behavior, not by your comments but by your own actions. This is when you offer to read her a story without being asked or when you invite her on a walk to a tea party while your husband minds the twins.

The child who has 10 minutes of time alone with each parent each day will have a basis for contentment. This is one of the secrets of good discipline.

There are some small tricks, too.

Your little girl will lower her voice automatically if you whisper, and she'll probably stop crying if you cuddle her in front of a mirror.

The biting can be controlled if you use a stern, low, no-nonsense voice as you put her little hand in her little mouth and give her little jaw a gentle, upward push. Even though you won't let her bite herself as hard as she bites others, it will be hard enough to give her the message, if not the first time than the second or third.

As much as you might want to slap her face when she bites, this would be much more than bad manners. This is not only the ultimate humiliation but it could escalate. Promise yourself that you won't do that again.

A slap or two on the fanny with a bare hand is the most a young child deserves. If it's given infrequently, it will be effective. Too much love can't spoil a child, but too many punishments can, for a child who needs attention will take it on any terms.

Your child isn't the only one who should have a few rewards. Three children in two years is a terrific stress on parents.

You need some free time and on a regular basis; anticipation makes pleasure all the sweeter. It may be a few hours of maid and child-care service each week -- even if you do telephone marketing at night to pay for it -- or a Mother's Day Out program at a neighborhood church or a commitment to yourself that you'll take a daily nap when the children do.

You and your husband also need to spend some time alone, to keep your relationship strong.

Parcel your children out to friends for a weekend every season, so you can get away together or even stay at home. There's nothing like a few days away from the children to make you appreciate them.