For most of us, fear of flab is the reason we exercise, the motivation that drives us to the gym. It’s also why public-health authorities have taken to encouraging ever more exercise as part of a healthy lifestyle. If we’re fat or fatter than ideal, we work out. Burn calories. Expend energy. Still fat? Burn more. The dietary guidelines of the U.S. Department of Agriculture, for instance, now recommend that we engage in up to 60 minutes daily of “moderate to vigorous intensity” physical activity just to maintain weight—that is, keep us from fattening further. Considering the ubiquity of the message, the hold it has on our lives, and the elegant simplicity of the notion—burn calories, lose weight—wouldn’t it be nice to believe it were true? The catch is that science suggests it’s not, and so the answer to all of the above quiz questions is “no”.