For most of us, the calorie is just a number on the back of the packet or on the display at the gym. But what is it, exactly? And how did we end up with this one unit with which to measure our food? Is a calorie the same no matter what type of food it comes from? And is one calorie for you exactly the same as one calorie for me? To find out, we visit the special rooms scientists use to measure how many calories we burn, and the labs where researchers are discovering that the calorie is broken. And we pose the question: If not the calorie, then what?