adolescents

Teenagers put a lot of stock in what their peers are doing, and parents are forever trying to push back against that influence. But with the advent of social media, hanging out with the wrong crowd can include not just classmates, but teenagers thousands of miles away on Instagram, Snapchat and Facebook.

Sure, you think, my kid's on a football team. That takes care of his exercise needs, right? Probably not.

"There are these bursts of activity," says Jim Sallis, a professor of family and preventive medicine at the University of California, San Diego. "But if you think about it, one hour of playing football out on the field means that the vast majority of that time is spent standing around waiting for the next play."