Most fat cells are called white fat cells - they store excess energy and make it tough to lose weight. But we've discovered how to turn mice's white fat cells into energy burning brown fat cells, and humans could be next. Weight loss isn't just about shifting mass - it's about finding ways to burn more energy through activity than we consume from eating. Those activities can (and should) include exercise and other physical exertion on a macro scale, but our individual cells are also capable of pitching in on the fight against weight gain. Brown fat cells are particularly good at this, as they naturally burn off excess energy instead of storing it.