Joy Passatta loaded her two children, a 12-year-old daughter and a 6-year-old son, onto their bikes on a hot summer morning. They rode a mile to swim class, where the kids burned off more energy in the pool. Then they biked home.

Later, she had more active plans: a T-ball game and a one-mile walk with the dog.

And despite all that, Passatta, 37, still worries her kids don't play outside like she did growing up.

"We rode bikes everywhere, and we knew everyone in the neighborhood," the Roseville, Mich., resident said. "At night, you'd hear my mom scream down the block, 'Hey, the street lights are on, come in already!' You don't hear moms screaming for kids to come home anymore."

It's no secret that even in the warm, sunny days of summer, children don't play outside like they used to.

U.S. children spend 50 percent less time outdoors than they did 20 years ago, says the Institute for Social Research at the University of Michigan.

The lack of running or biking or splashing around in the sprinkler is one likely factor in rising childhood obesity rates, said education professor Rhonda Clements, who conducted a 2004 study, "An Investigation on the Status of Outdoor Play," for Manhattanville College in Purchase, N.Y. In it, 85 percent of mothers reported their children spend less time playing outside than they did growing up.

What's keeping kids indoors? No doubt, technology plays a part.

With video games, hundreds of TV channels, DVDs delivered to your doorstep, endless Internet browsing and clips of hilarious cats on YouTube, kids have many more entertainment options than previous generations.

But Clements said fear plays a role, too. With tragic headlines drawn from around the nation, parents are often reminded about the danger of letting kids roam free.

"You don't want to become the hovering parent," Clements said, "so protective and fearful of letting kids go outdoors and making use of play spaces."