The Skinny on Obesity: Now here's someone with a brilliant idea. Instead of trying to find out why we gain weight so easily (which most of us do), find out why skinny people don't:

In the past, when I studied obesity, the thin rats were just a control group. But what I realized was that regaining energy balance following a period of overfeeding is a very dynamic process. Obesity is the more passive process. An animal or person overfeeding -- if they don't respond to that overfeeding, if nothing changes -- then they just keep gaining weight.