Metabolism
We used to think of weight gain or loss as only a function of calories ingested and then burned. Take in more calories than you burn, gain weight; burn more calories than you ingest, lose weight. But now we know the equation isn't that simple.

Obesity researchers now talk about a theory called the "set point," a sort of thermostat in the brain that makes people resistant to either weight gain or loss. If you try to override the set point by drastically cutting your calorie intake, your brain responds by lowering metabolism and slowing activity. You then gain back any weight you lost.