T-ball. Bike rides. Ballet classes. These tend to be the things we grown-ups think of as healthy doses of exercise for our kids. But it turns out that simply setting them loose on the playground or even just in the backyard may be even better for them. Researchers in Wales recently found that the way kids naturally play -- with quick bursts of energy, like chasing a friend across the park, interspersed with regular old goofing around -- is more beneficial for heart health and weight control than moderate, sustained exercise (like that bike ride). Alternating intense intervals with easier ones also gets kids fitter, faster. According to study author Julien Baker, Ph.D., six short sprints of up to 30 seconds each, done three times a week, can have the same cardio benefits and calorie-burning powers as five 45-minute sessions of more traditional exercise. (It's a technique worth stealing for your own workouts, too. The study was actually done on adults, inspired by previous research with school-age kids.) That's not to say it's time to toss the bike (or bat or tutu), but it's almost enough to make you reconsider schlepping kids and gear to and from all those practices. Why bother when you can just swing by the park?