If you want to impress at a cocktail party, bust this term out. It’s just a fancy word for fat cells, which come in two varieties: brown and white. Brown adipose tissue (BAT), which most animals have, is also called “good” fat because it stores energy as small units that burn easily — think of kindling in a campfire. White adipose tissue (WAT), though, is like a log: It’s harder to burn because it stores energy as one big unit. Our bodies can transform WAT to BAT. Unfortunately, it requires intense, prolonged stress, such as sustaining a severe burn.