Ever play pat-a-cake? Peekaboo? Do you gently lift your child up in the air, bounce her on your knee, or just sit quietly and rock? These "silly little games" are helping to lay a crucial foundation of physical, cognitive and social skills without which your child would be unable to function normally in the world.

In their first 2 1/2 years, children change more rapidly than at any other point in their lives. They grow from helpless infants dominated by their reflexes and environments to thinking, reasoning individuals able to control themselves and the world around them.

Swiss child psychologist Jean Piaget labeled these first years the "sensory motor" period because infants learn to make these changes primarily through their senses and physical activities.

Luckily for parents, nature doesn't require them to understand the complex hierarchy of developmental stages their children undergo. Instead it has given them an instinct for fun. The silly little games that parents commonly play are rooted in sound developmental strategy -- which may explain why they've been handed down from generation to generation.

The basic skills nurtured by these seemingly silly games have great implications for later learning. For example, around 6 months of age a baby will bang her hand aimlessly on her highchair tray. Frequently the parent will encourage this game by banging back, laughing, or saying "Bang, bang, bang!" By 7 months, the baby will have gained enough arm control to aim for and hit objects in her banging.

Around 8 months she will be able to grade her movements so she can bang hard and bang soft. This progression of skills has set the stage for more complicated tasks like bringing two toys close together without touching, gently placing a glass of water on a tabletop, putting balls in bowls, placing a clothespin in a bottle, and other activities requiring graded movements and delicate control.

Long before scientists "discovered" developmental sequences such as these, humans seemed intuitively to realize their existence and incorporate them into games so common you may not even recognize their importance.

Ask any mother why she rocks and cuddles, strokes, tickles, gently jiggles and sways her newborn. She's not likely to respond, "Why, I'm helping organize his vestibular, proprioceptive and tactile systems, of course." But she is.

The gentle motions of this earliest and most intuitive of all games -- rhythmic swaying up and down and back and forth, soothing touch, the deep pressure of being held tight -- help the baby control what have been called "the three greatest sensory systems in the body." The tactile sense is a response to touch; the vestibular, a response to movement; and the proprioceptive, a response to where the body is located in space. Being rocked, hugged and cuddled by you are your baby's first experiences with these crucial senses.

"In order to survive, each of us has to respond to these three systems first, and cognitive stimuli second," says Marsha Dunn Klein, a pediatric occupational therapist and author of several books on early developmental skills.

"A baby has to learn to organize and prioritize the information coming in from these senses. Once he has a base to work from, then he can learn more advanced skills. For example, when young babies reach for a toy, they may tip over because they haven't got their vestibular and proprioceptive systems together, and so they spend all their energy just trying to keep their balance. Once these systems are settled, the brain is able to learn other visual, auditory, and thinking tasks."

Ever watch an infant smile? Not only the mouth but the arms, legs, and body get into the act, wiggling and waving in a charming all-body grin. Babies experience their bodies as a whole and initially are unable to move one part without moving all the others. One of the first things they must learn to do is to "disassociate" or become aware of and control each body part separately.

Tactile and proprioceptive games like "so big!" that single out body parts for touch and movement help children realize, "Oh -- those are my hands. That's my toe. Those are my arms and that's what they feel like over my head." As they gain a sense of where their body parts begin and end, children become able to move them separately, which of course is a basic skill necessary for many more complicated tasks.

That old standby pat-a-cake represents a neurological milestone of sorts. When children are able to bring their own hands together in the center it indicates that the hemispheres of their brain are beginning to coordinate across the brain's midline, or corpus callosum. In other words, they're learning to control the two sides of their bodies separately and together. This coordination must be present for a child to play with both hands, transfer an object from hand to hand, and do different things with each hand.

"This little piggy" and raspberries (or motor lips or whatever you call the sputtering blast parents like to blow against a baby's tummy) not only aid in disassociation but also help build the concept of anticipation. This in turn is related to the concept of cause and effect, which is one of the fundamentals of abstract thought: If this happens, then that will happen.

Even the belly laughs from these have an ulterior purpose. They help strengthen the muscles used in crawling, expand lung capacity, and give the baby experience in making noises of different pitch, one of the precursors of speech.

At the same time as he is discovering his body parts, a baby is gaining the muscular strength necessary to use them. One of the most crucial strengths, necessary for all other coordinated movements, is head control. It's no surprise that parents seem to begin head control games almost immediately.

Every time you bounce your baby on your lap, ride her on your crossed leg (ride-a-cockhorse), or lie on your back and hold her above you, you are helping her strengthen her head control.

Once babies have begun to sort out their body parts and senses, they need to put them together in useful ways. Being able to coordinate sight, sound, reach and grasp leads to the ability to manipulate objects.

Because babies enjoy them so much, parents tend to play a lot of watch-reach-grasp games in the first eight months, about the time it takes to master this skill. A musical mobile hung over the crib helps strengthen focusing, tracking, and depth perception and helps babies find with their eyes what they hear with their ears. Shaking a rattle entices a child to swing out at the attractive item.

Consequently, she gains skills in making her arms go where her eyes want them to go; and when she hits the rattle, she learns how to grab and hold on (learning to let go starts at about six months.)

Watch-reach-grasp coordination is needed before a child can color, draw or write with a pencil. Researchers also believe that the experience babies get in manipulating small toys of different shapes helps form concepts necessary for later development of technical and mathematical skills.

As a baby gains expertise in watch-reach-grasp coordination, he is able to make more and more delicate manipulations with his fingers -- and suddenly the whole world opens up. Being able to obtain objects at will and examine them in depth gives children the independence to control their own experiences. Now that they have the tools to manipulate their environments, abstract thought can begin.

True knowledge comes from action upon objects, Piaget said. In this light, we know that empty-the-cupboards is really a scientific experiment.

As the pots and pans come bouncing and clanging out of the cupboard, children are researching such concepts as: the basic properties of objects, cause and effect, classifying and ordering, problem solving, and practical thinking. In sitting, crawling, standing and reaching into the cupboard, children learn about space, distance and direction -- lessons necessary for understanding more complicated concepts like left and right, near and far.

Long after pat-a-cake and "so big!" have been dropped, parents and kids still love playing peekaboo together. In the tradition of all great games, this can be played in hundreds of new ways, growing in complexity with the players' abilities. This early version of hide-and-seek has far-reaching implications for development.

Peekaboo nurtures "object permanence" -- the knowledge that objects continue to exist when out of sight. The game grows with the child. In its earliest form, Mom or Dad cover their face (or the baby's). The infant is highly motivated to investigate the disappearance of the most important object in this world -- his parent. As he learns that Mom or Dad did not in fact "disappear," he progresses (beginning at seven months) to searching for hidden toys and still later to hiding himself and leaping out with a triumphant "Boo!"

As the game develops, children learn to create and retain a mental image of something they can't see -- no more "out of sight, out of mind." This ability underlies the beginning of memory and the beginning of speech (using abstract symbols to represent an actual object). Next, children begin to make mental projections about what has happened to objects they cannot see -- another basis of abstract thought. Not bad for such a simple game.

Imitation is so important to all areas of development that babies do it from birth.

On the purely mechanical level, imitation teaches children how to do something -- how to say a word to bounce a ball. But imitation also teaches intangible cognitive and social skills. When a baby imitates something she has seen, like pretending to stir something in a pot, she is learning to think symbolically. Through imitation, she begins to make sense of the world and gains a feeling of mastery and control. As she internalizes chunks of behavior and pulls them out to imitate at will, she forms a sense of self-identity. No wonder "let's pretend" is the most popular game children play, especially with their parents.

Learning to imitate actions lays the basis for learning complex social skills like empathy and following directions, and ultimately leads a child out of her sensory mode of relating to the world. This development helps her channel her boundless energy into productive ends.

But parents know play is much more than a developmental tool. Fun and games nourish the roots of love, trust, security and friendship every human needs to survive.

Just as babies need a basic sense of their bodies to learn, they need a basic sense of trust in the world. Without it, they become insecure and fearful, hesitant to take the risks needed to learn.

"More than anything else, play is likely to elicit an 'I love you'," observes nurse Darlene Curtis, head of Journeys ... From Stress, a parent-support group in Tucson, Ariz. "It's as if play is an opportunity for children's lovability to come out. Children understand an 'I love you' long before they understand words. When you play with them, they can feel it." Chris Medvescek, a free-lance writer in Tucson, Ariz., is the mother of 3 1/2-year-old Sarah. Playing these "silly" games with Sarah, who has cerebral palsy, has provided important clues to her development.