To measure how many calories there are in food, we use a hot little device called a bomb calorimeter.
It burns foods and measures the difference between how much energy went in versus how much energy comes out.
Here’s where it gets confusing if you ever talk to a physicist about your weight loss program: What we call a “calorie” is actually equal to a thousand scientific calories, or a kilocalorie.
So to a scientist, a doughnut doesn’t actually have 235 calories, but 235 kilocalories (235,000 calories). You’d better start jogging.