If we want to fight childhood obesity, we need to get kids moving.

Sounds incredibly obvious. And that was my reaction when I first read the study just released in the journal Pediatrics—in it, the authors said that high school students were less likely to be overweight or obese if they played at least two sports during the school year, or if they walked or biked to school.  Really? Next we will show that if you study you are more likely to do better on tests—or that it’s easier to drink milk if you open the carton.

But after thinking about it for a while, I’ve decided that when it comes to exercise, stating the obvious is really important.

We are in the midst of an obesity epidemic. Currently a third of US kids are overweight or obese; in adults, that number is two thirds. And physical inactivity is a big part of that. Just this week, in advance of the Olympics, the Lancet released a series of articles on the effect that physical inactivity has on our health. They report that physical inactivity causes around one in ten deaths worldwide, which is about the same as smoking. Not only that, four out of five adolescents are at high risk of disease from failing to do recommended amounts of physical activity.

Four out of five. If you have an adolescent, chances are that includes your kid. Is he or she moderately active for at least an hour a day?

What the Pediatrics study does is offer a practical idea: get all high school students playing at least two sports a year. As someone who was one of those last-picked-for-kickball geeks in high school, that sounds close to impossible. But it’s actually not. It would take a culture change (we’d have to think of sports in a more inclusive way, or make them mandatory) and it would cost some money, but it could be done.

The Lancet series tells us that we need to think of physical inactivity as a public health problem—and instead of just concentrating on getting individual people moving, we need to think about getting populations of people moving. Like the elderly, or poor, or people without safe places to exercise. Or high school students. Or all kids.

Some people are more likely to be physically active than others—and among them are those that have been physically active in the past. That makes the idea of getting youth involved in sports, whether it’s in high school or earlier, an even better one—because not only does it help them be healthy now, it builds habits that can keep them healthy for the rest of their lives.

See, it’s these practical ideas that we need. One of the Lancet articles talks about interventions that have been effective, like mass media campaigns, signage encouraging people to take the stairs instead of the elevator, walking clubs, free exercise classes in public places, or closing streets to traffic once a week for people to walk and bike.  It turns out that just sprucing up streets and getting better lighting can increase activity levels by up to 50 percent.

We just aren’t getting it when it comes to obesity and physical activity, somehow. We think that our kids will grow out of their baby fat. We think that when our lives get a little simpler next month or next year, we’ll start going to the gym or start running or start that diet. Even worse, we’re starting to think that it’s normal to have a belly or be sedentary; after all, so many people around us are just like us.

We got it when it came to smoking. We created smoking bans and taxes on cigarettes and educational campaigns—and cut cigarette smoking dramatically. Now we need to do the same thing to tackle obesity and inactivity.

You don’t want your kid to smoke because you don’t want them to get sick or die. Think of inactivity the same way. So get them moving. While you’re at it, get moving yourself. Join in the effort to get everyone moving.

Lives are at stake.

Claire McCarthy is a primary care physician and the medical director of Children’s Hospital Boston’s Martha Eliot Health Center.  She blogs at Thriving, the Children’s Hospital Boston blog, and Vector, the Children’s Hospital Boston science and clinical innovation blog.