How can we decrease the body's energy efficiency? The answer to this could be used to fight the exploding obesity crisis. Our ability to accumulate and retain energy reserves once provided a survival advantage. However, these ingrained energy-conservation pathways are now driving unprecedented weight gain in modern societies where calorie-dense food pervades. Burning off excess fuel (analogous to heating a house in winter with the windows open) may be an effective therapeutic avenue to reduce obesity when diet and exercise are not enough. On page 1158 in this issue, Vegiopoulos et al. demonstrate that the fatty acid derivatives called prostaglandins encourage adipocytes (fat cells) to do exactly this—waste energy through increased heat production .