How can parents and society keep kids from smoking? One way is to make sure that merchants know they'll get in trouble if they sell cigarettes to minors. That can cut by 21 percent the odds that a 10th grader will become a daily smoker. And that's good news, because the earlier a person starts smoking, the more likely he or she will be hooked for life.

Public-health advocates have been trying for years to figure out how best to keep children and teens from taking up the cigarette habit. It's often hard to determine what works and why. Banning sales to minors doesn't work unless the bans are enforced, more is needed to be done to keep kids from starting to smoke because it is hard as it is for patients who smoke to quit.

Parents have power over how well the ban on selling cigarettes to kids is enforced,  because much of the enforcement is at the local level, from the police and local health department. "They get pushback from the merchants," he says. "If they're getting kudos from parents on how great they're doing, they will make a greater effort."

But tough enforcement is not the only thing that works to prevent teen smoking. For instance, the increasing price of cigarettes reduced daily smoking by 47 percent over the same 1997-to-2003 period, the new study's authors found. Antismoking restaurant policies and advertising campaigns against tobacco helped, too. The new study is the first to separate the effect of enforcing a ban on sales from these other factors. Banning sales helps over time, says DiFranza, because younger children see fewer older kids smoking. Teenage smoking declined by half between 1997 and 2003.

But this doesn't mean that parents are off the hook. The biggest predictor of whether a kid will smoke is whether Mom and Dad do. If there was ever a good reason to quit, this is it, noting that tobacco causes more health problems and early deaths than all illegal drugs combined. It's full of good arguments against smoking, for kids and grown-ups alike.