Ever wonder how they determine how many calories are in food? Yael and Don explain it in this Moment of Science.   Y: All right, Don. What are you doing?

D: I'm using a bomb calorimeter to figure out how many calories are in my lunch. See, I put my sandwich in this sealed container, which I'm going to submerge in water, and then I'm going to burn my sandwich and figure out how much the temperature of the water has changed, because a calorie is defined as the amount of energy it takes to heat up a kilogram of water one degree centigrade--and then--Y: Whoa, Don, two things: First of all, you won't be able to eat your lunch once you burned it.

D: Oh, good point.

Y: Second, no one uses the bomb calorimeter anymore to measure the calorie content of food.

D: They don't?

Y: Nope. Today most of the nutritional information you find on food labels is generated indirectly, using what's known as the Atwater system. So basically, calories are calculated by adding up the calories provided by each of the food's energy- containing nutrients.

D: Oh, like proteins and fats?

Y: And carbohydrates and alcohol. According to the Atwater system, there are on average four calories in each gram of protein, nine in each gram of fat, seven in each gram of alcohol, and four in each gram of carbohydrates.

D: Oh, I've seen that conversion scale on the label on my potato chips.

Y: Also, because some carbs contain fiber that your body can't break down, their total is adjusted accordingly before figuring out calories. Atwater already figured all that out using your beloved bomb calorimeter, so now you don't have to.

D: Darn. I sure do love burning things!