Scientists have finally figured out how the key gene tied to obesity makes people fat, a major discovery that could open the door to an entirely new approach to the problem beyond diet and exercise. The work solves a big mystery: Since 2007, researchers have known that a gene called FTO was related to obesity, but they didn't know how, and could not tie it to appetite or other known factors. Now experiments reveal that a faulty version of the gene causes energy from food to be stored as fat rather than burned.