Ever wonder why it’s easy to call forth self-discipline one moment, but difficult in another? Several years ago, researcher Dr. Roy Baumeister, a professor of psychology at the University of Florida, pondered the same question. To understand why self-discipline can be elusive, Dr. Baumeister and his team ran an experiment: they wanted to know whether or not self-discipline was like a muscle—something that could be weakened with overuse. To test this question, they brought a group of hungry subjects into their lab and had each subject enter into a room with a bowl of cookies and a bowl of radishes on a table. They told half of the group not to eat the cookies, but instead to eat the radishes. The other group could eat whatever they wanted. (They all ate the cookies.) Then, immediately following this experience, the subjects were brought into another room, where they were asked to complete a complex math problem. In actuality, the math problem was insolvable—the researchers were actually measuring how long the subjects persevered in trying to complete it.