system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can something have no calories? What does the body do with this calorie free food?
gpt
Calories are a measurement of energy, which comes from breaking bonds between atoms — mostly, carbon atoms. Just like the gasoline your car uses that makes it run (gasoline is primarily carbon and hydrogen). When your body breaks down things like sugar, fat, and protein, it breaks the bonds in it and releases energy that it uses to do various things. 

Food has varying amounts of sugar, fat, and protein. A bag of Skittles is almost all sugar. A slab of meat is mostly protein, with some fat. All of those things have calories that come from those compounds, which your body breaks down. 

Some foods do not have anything that your body can break down. Water, for instance, has no carbon. It goes in and most of it goes out (in your pee or your sweat). Some foods, like diet soda, have carbon compounds in it, but they are designed so they can’t be used by your body (they create the taste of sweetness but can’t be broken like normal sugar, hence no calories). And other foods have carbon, but it’s also in a form you can’t use — like celery, for instance, which is mostly fiber. It has substance, but (mostly) not a kind your body can break down and use. If your body can’t use it, there’s no energy to gain from it and therefore no calories. 

Also, if you consume extra calories that your body doesn’t need at the moment, it uses that energy to make things to store and use later. One of those things is fat (your body can make fat). Then, if you go a while without eating, it will break that fat down and use it for energy. You don’t need to eat fat to make fat. Your body can use anything with calories (energy) to make fat. That’s why you can get fat even if you eat a fat free diet but eat a ton of sugar.