Wanting to exercise has never been so easy. Scientists have discovered that switching on the hormone leptin—which regulates appetite—doubled the amount of physical activity overweight mice did. The research suggests that scientists could potentially design medication that induces the will to exercise. When the hormone leptin was turned on in the morbidly obese, severely diabetic mice they reportedly ate fewer calories and lost a modest amount of weight. Researchers are now searching for a similar treatment in humans.