system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you lose weight, where does it go?

I mean, Law of Conservation of Mass. Do the fat cells just leave via your poop? Do they come out as sweat? Okay, I'm pretty sure it's not that. But where does it go?

Yea, I've taken Physics and Chemistry and Meteorology and other stuff, but not Biology (at least not since high school).
gpt
The fat cells actually never leave, until or unless that cell dies off naturally, as all cells eventually do. 

When you lose weight, you burn off the energy that the cell contains. This is like drinking the water out of a glass -- the glass is still there, but the content is less. 

This is why it's very easy for a formerly fat person to re-gain the weight. The cells are already there, and are already used to storing excess fat.