Play is a child’s work. As he plays, he has a chance to relive his experiences, ask questions about his world and, most of all, act out his dreams.

In play, he may feel the safety of not being watched or directed, and the freedom and exhilaration of enacting his own ideas. In play, a child can embellish his new developmental achievements, experimenting as he goes.

For example, when he has learned to walk, he may pick up a large wooden block that’s too heavy for him, drop it, and lean over to try to pick it up again. He may hold it in both hands this time, teetering as he concentrates. Losing his balance, he sits down hard but still holds onto the block.

Now he can turn it over, mouth it or push it to make it go, growling like a car engine. What has he learned?

1. To enlarge on the new task of walking

2. To balance while holding a heavy toy

3. To turn a wooden block in his imagination into a noisy car

In this one bit of play, we see an athlete, a scientist and a builder of dreams.

When does play begin? At each diapering, each feeding, each time he’s put to bed, the baby starts to respond to his parents’ rhythms, smiles and strokes.

By 4 months, play can become more complicated. A baby can add peekaboo or play with a toy strung over his crib. If he bats it, it will swing around.

The baby sees that he can have an effect on his world. This is a time when play can postpone more basic requests — such as crying to be fed. The baby learns to fill up his own space with independent play. A parent can begin to push him into a schedule.

Much of play is to test how the world works. By 7 to 8 months, he can crawl toward a forbidden TV or lamp. As he advances, he looks back to check his parents’ watchfulness.

Then a parent rushes over to pick him up. He is learning to predict and control important adults around him. He tries his maneuver again. His mother drops the phone to come to him. He squeals with delight. What a source of power!

Once a child can walk, all kinds of new experiments are possible. He can walk around the corner and out of sight of his parents. If that doesn’t bring them, he may screech, partly afraid he has lost them, partly to get a response. When his parents rush to him, he has learned more about himself and them.

By 14 months, one toddler may sit beside another. One of them picks up a block to shake it. Without seeming to look, the other shakes his block in the same motion. Their play becomes matching. They try out rhythms, hiding the toy, throwing it, testing the friend by stealing his toy.

They are starting to explore social skills, communicating without words, joining and not joining a friend.

By 18 months, a child will imitate much of the world around him. He takes a teddy bear and cuddles it. He wraps it up in a blanket. He has taken the step into symbolic play. He play-acts what he has experienced in his own nurturing.

At 3, a child can even try out a variety of grown-up roles. A little girl might put on her mother’s jacket like a dress. She has incorporated her mother’s femininity and is trying herself out as a grown-up woman.

A boy will do the same with his father or an older brother. Fantasies are thus not an escape from the world but an exploration of how it works.

As children grow older and play more elaborate games, they learn about rules. Even here they experiment, and they may try to cheat to see the reaction it brings. They also learn to play cooperatively: Building forts, playing hide and seek, or acting out a story all require working together.

It’s easy to see how a heavy dose of television or video games can usurp some of the learning and joy of free play. It substitutes ready-made fantasies and passive watching for independent, active exploration and freewheeling imagination.

The extra richness and freedom of outdoor play, with its endless discoveries and new sensations, can be a high point of childhood. There the child can step fully into the roles of explorer, experimenter, builder and dreamer.

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