CHICAGO — Donna Yehl's fourth-grade students bob behind their desks, heads nodding up and down as if the children were on the deck of a ship.
But they aren't fidgeting.
The two dozen children in Yehl's Elgin, Ill., classroom read and write -- in fact, do all of their classwork -- perched on exercise balls.
The inflatable balls are commonly used in Pilates, yoga and exercise classes. Some teachers say they belong in school classrooms too because they sharpen students' attention and improve their posture.
This year, Yehl checked the Internet for ways to help her restless pupils sit still. She stumbled on a story about exercise balls improving concentration. So she replaced her classroom's chairs with bouncy 21-inch-high balls in colors students chose.
She sees a difference.
"They're more focused," Yehl says. "They're sitting upright."
Balls and ball chairs, which have wheels, feet and backs, are popping up in classrooms.
Lisa Witt, whose Wisconsin company WittFitt sells exercise ball chairs for classroom use, reports a sharp increase in customers, from one school in 2004 to more than 300 across the country and abroad.
"Some people initially think, 'Are you crazy?' " says Witt, a former elementary school teacher in Colorado. But aside from mental and physical benefits, she says, "it's just plain fun."
Kids often agree.
Emily Ziemba, 10, a student in Yehl's class, laughs and nods when asked whether she likes sitting on an exercise ball.
"I mean, sometimes I would like to lean back," she says. "But other times, it's better than a chair."
Teachers say children on ball chairs often quit flopping over on their desks. And they're getting enough exercise to improve concentration.
"You'd be surprised how many kids really need to move while learning," says Adrienne O'Brien, a fourth-grade teacher in Barrington, Ill., about 40 miles from Chicago, who calls her more energetic kids "squirrels." "That would be the majority of them, frankly."
Subconscious mental activity lies at the core of the science behind the balls' success, experts say.
The tiny movements kids make to stay balanced stimulate their brains and help them focus, says Dr. John Ratey, a Harvard University professor and author of "Driven to Distraction" and "Spark: The Revolutionary New Science of Exercise and the Brain."
Children with attention disorders, he says, have "a sleepy cortex," and exercise combats that mental disengagement.
"Just by using their core muscles more, they're flipping [their cortex] on" and increasing their mental activity, Ratey says. "The cerebellum part of their brain is really working to adjust, every millisecond."
John Kilbourne, a professor of movement science at Grand Valley State University in Michigan, experimented with college students last year. He found that they preferred ball chairs to regular chairs and that their concentration improved while balancing.
In elementary school, though, some students equate exercise balls with playtime. So teachers devise ways to keep them from goofing off.
Yehl begins the school day by playing music and letting the kids do what she calls "baby bounces": They move up and down but keep their feet on the floor. She follows the same routine after recess.
Kids sometimes sway back and forth or roll on the balls, she says, but they rarely leave their seats.
And during class, the room is strikingly quiet.
"Even teachers walking past the room said they'd never realize [students] were sitting on exercise balls," Yehl says.