It used to be that preschool and kindergarten were all about play. Children spent their day molding clay, dressing up, and running around outside.

Now, kindergarten is the new first grade with much more of the school day spent on reading readiness and learning how to add and subtract.

When did "play" become a dirty word?

In the early '90s, American schools started feeling the pressure. Our students were falling behind the rest of the world, we were told. We needed to kick it up a notch.

So schools crammed more and more into the curriculum. Something had to give, and it was often "playtime:" recess, the arts, and other nonacademic pursuits.

"We called it a constipated curriculum," says Janice Sutter, a kindergarten teacher at Sylvania's Whiteford Elementary School. "They put a lot more in and nothing was coming out. Time kept being eaten away."

But experts warn that leaving play out of a young child's school day is not the answer. "Play is important because it helps to build skills that everyone's looking for in 21st-century learning. It doesn't take away from their learning," says Kathy Hirsh-Pasek, professor of psychology at Temple University and co-author of Einstein Didn't Use Flash Cards (Rodale, 2003). "It's more consistent with how the human brain works."

Play, in fact, is the way young children learn. It sparks creativity and critical thinking and helps kids make sense of their world. They solve problems, feel power, and learn social skills.

Think about those games of kickball you used to play. You learned without realizing it. You picked teams, negotiated the rules, played fairly (or not). All that "play" translates into valuable life skills that can help children in core subjects like reading, math, and science, Ms. Hirsh-Pasek says.

One study that she worked on found that children who attended more academic preschools had no long-term academic advantage over children attending more play-oriented schools. In fact, children in more academic schools appeared to be less creative and less enthusiastic about learning.

The added emphasis on testing has also taken its toll on playtime. Teachers didn't used to worry so much about when a child "got it," Ms. Sutter says. They knew they'd get there eventually. Now "testing has really sucked the joy right out of it," she says. "The current test-taking fervor has changed the game a lot."

The way kids play has changed, too. Kids used to go home after school and play tag or collect leaves, says Rusland Slutsky, associate professor in the University of Toledo's Department of Early Childhood, Physical and Special Education.

Now we "pay to play," signing up our kids for sports teams, karate, and science camps. "We're paying for children to have social experiences," he says. "Kids have lost a lot of that social power."

They no longer are the driving force behind their own play. Adults are telling them what to do and when. Dolls have their own biographies. New versions of Monopoly count the money for you and even roll the dice. You don't even have to find someone to play with if you're competing against the computer.

"They've lost the ability to creatively engage in play because so much is directed," Mr. Slutsky says. "Toys do everything for them."

He says schools are being called on to pick up some of the slack. If children don't get opportunities to engage in social play at home, they need it even more at school.

As a teacher in the trenches, Ms. Sutter knows her students don't play as much outside or at home. In fact, this changing dynamic forced her to rethink her stance about full-day kindergarten.

When she started teaching kindergarten in the '80s she was skeptical of full-day classes. "I wanted [the students] to interact with their parent at home," she says.

But she realized that wasn't happening. More women were working. Children were left with caretakers or at day care.

When Sylvania kindergarten went to full days in 2010, she realized the extra time was a boon, a way to put more of that valuable playtime back into her students' days.

Now her kids get a full hour of free-choice play in the afternoons -- and that amount of time is huge. Kids need time to develop play, to let their stories and ideas play out. They need time to test their hypotheses on how best to build that block tower.

They need time to just be with each other. "Kids learn better from each other," Ms. Sutter says. "Talking and interaction are still the most powerful thing in my classroom. And you have to have play time to do that."

In her classroom, she can have a boy who just turned 5 playing beside a boy a full year older. That's a big difference at that age. One may be using the blocks to build a complicated structure; the other may be pretending the block is a car.

But through play, the two can sit next to each other, use the same toys at different levels, and learn from each other, she says.

Is all play equal? No. Experts talk about "quality" play. First, children need to be the boss of their own play and have the latitude to do things their way.

Have you ever witnessed a child turning a simple paper cup into a rocket ship or a boat sailing into stormy waters? This ability to think imaginatively is a valuable 21st-century skill, Ms. Hirsh-Pasek says. "Steve Jobs thought of things no one else thought of," she says. "If someone had told him to do a job in a certain way we wouldn't have the iPhone today."

Equally important, though, is having an adult to observe, guide, and expand on a child's play. Ms. Hirsh-Pasek says research shows that the level of a child's play rises when an adult joins in. Not when the adult controls, but when she joins in.

Kids -- and really all of us -- learn better when activities are active, engaging, and meaningful, Ms. Hirsh-Pasek says. "[Learning is] not just memorization and worksheets," she says. "Learning exists in the school day and beyond it. It's not something that merely happens in the 20 percent of time we're sitting in desks in rows."

First Published November 13, 2011, 5:15 a.m.