system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When breathing turns oxygen into carbon dioxide, where does the carbon come from?
gpt
The food you eat. Sugars, fats, carbohydrates, they are pretty much just variations of long carbon/hydrogen based molecules.

Inside your body you react them (basically set them in fire without the flame) and they break down into CO2, water, and heat/energy. That heat/energy keeps you alive and moving, the water keeps you alive (and you eventually pee or sweat it out, and the CO2 you breath out.

There are a lot of mini steps along the way but thats the gist of it.