system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you burn fat on your body by exercise etc., does your body burn newer fat, or any fat?

If you only burn newer fat, does that mean that there is some really old fat in your body?
gpt
Your body has a fixed number of fat cells that store liquid fat inside of them. When you burn fat what's happening is your fat cells are being told to release energy in to your bloodstream. So it's sort of like if you had a bucket of water that you sometimes add water to and sometimes take water out of. Technically some of the water molecules may have been in there a long time just like how some of the fat molecules may also have been, but there would be no way to know exactly.