Q. "I have a 21-month-old boy who seems very bright for his age.

"He has always been very good, but now he is starting to slap me if he's mad at the least little thing. This happens even though I don't hit him to discipline him.

"The main problem is trying to get him to sleep. He won't stay in his crib so we put him in a big bed. He seems to like it, but won't stay in it.

"For naps I have a cloth strap that hooks onto the bed frame and around his waist to keep him still until he falls asleep.

"Is this a stage or is there something I can do?"

A. As parents we all have one thing in common: We take wrong turns sometimes. Welcome to the club.

In this case you've let your child negotiate terms that you should have set yourself. The conflict is inevitable, surrender is not.

Your child is born to grow up and he takes bigger and bigger leaps for independence at 15 months, 18 months, 24 months, and then at what will be the nadir for you -- 26 months. The more you try to keep him in place now, the more negatively he will react. Yet he needs discipline at every age, for his sake as much as yours.

It's a matter of changing your style to suit his new growth.

Change the naptime, and please: Don't tie him to the bed. This not only may be dangerous, but the restriction is a minute-by-minute reminder that you're trying to control his body.

Now, for the first time in his life, he can make his arms and legs move the way he wants. This is an extraordinary breakthrough and the source of his independence. If you're worried that he may fall out of the bed, push one side against the wall and put a bed rail on the other, or lay pillows on the floor to catch him if he falls.

To undo bad habits, give hugs and kisses, read a story and take him to his room, explaining that nap times are now quiet times. He can sleep or play quietly, but must stay in his room. And he stays there because you've put your husband's sock on the door knob with a rubber band -- so he can't turn the knob.

Have him help you set the alarm clock and put it outside his door. Tell him you'll come get him when it rings. In the meantime you'll be resting so you won't answer if he yells. And if he does scream (and he wll), keep your word. This is the hardest part, but your child will learn that naptime is no longer negotiable.

When the bell rings, go in with a kiss and a "how-nice-to-see-you" as if he hasn't been yelling at all. Pick up all the toys he's thrown on the floor without a word, since you're only teaching him one thing at a time.

In a week or so he will stop shrieking and start playing and in a month (or six), he sill be less rebellious and willing to nap. It doesn't matter where, so long as it's in his room. The first few times he sleeps, take him some place special, like the zoo, because he has so much energy from such a good nap.

To promise an adventure beforehand would be a bribe; to deny it unless he naps would be a threat; to give it afterward, unexpectedly, is behavior modification. This is the one that works.

The same principle applies at night. Give him 10 minutes of attention, a new lullaby or some poetry, letting the timer enforce the cutoff. He is then tucked in bed, the sock is hung on the doorknob and he probably howls. This should be over in about a week (or two or three). In the meantime don't invite company to play cards.

How soon he accepts the new rules will depend on how well you relax some of your own. If you write down every time you say "no," you'll find there are many unnecessary no's, including those that could have been said once or twice -- instead of 5 to 10. The less you correct your child, the less he will rebel.

And as for slapping you: Don't permit it. Tell him "my friends don't hit me," and ignore him for maybe 10 minutes. You don't answer, you don't slap back; you don't engage, but when he's good, you tell him how much you enjoy his company. A smart little boy learns that he gets attention for being good instead of bad.

A "Terrible Two" -- or a 21-monther -- can be pretty terrific. Like a teen-ager, he gives back respect so long as he gets it.

It's just that he wants so much.