“Let us begin with a short quiz: a few questions to ponder during the 30 (or 60 or 90) minutes a day you spend burning off excess calories at the gym, or perhaps while feeling guilty because you’re not so engaged. If lean people are more physically active than fat people — one fact in the often-murky science of weight control that’s been established beyond reasonable doubt — does that mean that working out will make a fat person lean? Does it mean that sitting around will make a lean person fat?” (09/27/07)