You know what it takes to forgo dessert or resist the urge to buy that bauble you can't afford: self-control. That sounds simple, but self-control can be a slippery thing. A study in the current issue of Alcoholism: Clinical and Experimental Research sheds some light on why. According to the study's author, Roy Baumeister, a social psychologist at Florida State University, self-control is neither an acquired skill nor a logical cognitive process. Rather, he says, it's an exhaustible resource that operates like a well: it is emptied with use and refilled with rest. To test this theory, Baumeister gave subjects a variety of exercises designed to tax their self-control. In one of them, a group was asked to refrain from thinking about a white bear (not an easy thing to do once the idea has been planted in your mind). The other participants were allowed to let their thoughts wander. Afterward, both groups were given tricky anagrams to solve. The white-bear folks gave up much faster than the free thinkers, suggesting that the former had depleted their supply of self-control. Though the precise mechanism of willpower is unknown, Baumeister suggests that it can be restored by sleep, positive emotional experiences or self-regulatory exercises such as monitoring your posture or keeping a food journal.