Babies need parents to keep saying “no” until a lesson has been learned and no longer needs to be tested.

Parents are understandably frustrated when a baby or toddler engages in the same behavior over and over again to elicit a “no.” Since the baby already knows what the rule is, he seems to enjoy tormenting his parents.

Why must every child test? They test because they are trying to understand: Is it always “no”? Is it “no” when you say it this way — or that way? Is it “no” in the kitchen too? Is it “no” when my friends are over or just when we’re alone? Perhaps it is “no” the first, second and third time, but not the fourth and fifth. The baby may wonder: Perhaps my persistence will pay off if I try just once more!

To make matters more confusing, sometimes “no” doesn’t apply in every context. Early on, the television controls all seem the same: They turn or push, and Mommy gets mad.

But a little later, the baby or toddler will see that one button makes the exciting box shriek and boom, another produces a new picture and a third switches everything off. Is it the same “no” for all of these?

“No, you cannot sit in my lap at suppertime.” True at home, but not in a restaurant without a high chair, and not at the grandparents’ house, where the child is bound to squirm his way out of his portable booster seat to explore exciting new territory, un-childproofed and full of breakable treasures.

“No” may not be the same for one child as for other siblings. The baby who crawls toward the television will have to learn that the buttons are off-limits for him. He will have to understand that the rules are not always the same for everybody. (Even parents struggle to accept that fairness doesn’t mean that the same rules always apply to everyone. In some instances, each child in a family should have different rules, in keeping with individual needs and abilities.)

Add the inevitable fact that rules and expectations will change as the child grows.

“Yes, you can turn the volume knob now that you know how to do it!”

“Wow!” this child may think. “Maybe I can handle those knobs on the stove now too!”

Then there is the likelihood that “no” from different adults will not sound or look alike. From a parent who works at home, “no” may sound worn out, but from a parent who is away all day, “no” may sound less humdrum and is likely to command more attention. An older sibling’s “no” may sound more raucous, and a grandparent’s “no” may come as a complete surprise!

A home-all-day parent may say “no” to roughhousing right before bedtime, whereas one who has just returned from work may relish it. The parent who worries more about safety may say “no” to rough-and-tumble play. But if the other parent feels differently about such play, a child is bound to read the nonverbal approval of bright eyes and a quiet smile.

Parents of babies and toddlers are often just discovering differences in each other’s expectations for their children. Mixed messages are the understandable result.

When we stop and consider just how much we’re asking young children to learn, it’s easy to see why repetition, consistency and clear messages are so important for them.

MAKING RULES CLEAR AND CONSISTENT

1. Decide what your rules are.

2. Make sure you and the other parent agree.

3. Adapt your rules to each child’s needs and abilities: They needn’t be the same for everyone, and you can help each child understand why that’s fair.

4. Tell your child what the rule is, with words, tone of voice, gestures and facial expression.

5. Expect your child to test you.

6. Respond the same way each time. Any variation makes the child curious to see what will happen next time.

7. Expect your child’s new abilities to take you by surprise.

8. Plan on reassessing your rules and expectations regularly. As your child grows, you will need to adjust.

Questions or comments should be addressed to Dr. T. Berry Brazelton and Dr. Joshua Sparrow, care of The New York Times Syndicate, 620 Eighth Ave., 5th Floor, New York, N.Y. 10018. Questions may also be sent by email to:

nytsyn-families@nytimes.com