system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: who and how do they determine how many calories are in a particular item of food?
gpt
A calorie in your food is the amount of energy that is needed to raise the temperature of one kilogram of water by 1 degree Celsius. 
This is done by burning the food in something called a bomb calorimeter. That is a box within a box. A sample of food is put on a dish in the inner chamber of the calorimeter, fill it with oxygen and seal it. In the outer chamber a measured amount of water is filled. An electric spark in the inner chamber ignites the food. As it burns it raises the water temperature in the outer chamber. The amount the water heats tells us how many calories are in the food. If the the temperature of the water goes up one degree it has one calorie.