It flew off the shelves last year, partly because some parents wanted their video-game-obsessed kids to finally get a workout. But now that the interactive Wii Fit has become mainstream, some wonder if it’s effective. “If Wii Fit is the only activity a child gets, then it might be a good tool,” says David Dzewaltowski, PhD, a professor of kinesiology at Kansas State University in Manhattan. “But children are likely to get better exercise from being outside and playing games that require using large muscle groups.” He also warns that the body mass index calculation on the Wii Fit is a good measure for adults but doesn’t apply correctly to kids because of their changing height and weight.