system
Given the following post, respond as if you're commenting on Reddit:
human
How does "burning fat" physically work?

Like, do the fat molecules actually just fall off your body and into the environment? Meaning that gym floors must be filled with people's invisible ex-fat (I thought of this question while working out)?
gpt
When fat or sugar are burned, the result is CO2 and H2O. The CO2 exits your body when you breathe. The H2O hangs around in the blood, but will leave the body through various means.

An average 140 lb person will lose about 1 kg of CO2 per day from basal metabolism. That takes about 270 grams of carbon out of your body. Fat is mostly chains of CH2, so about 85% carbon, and the rest mostly hydrogen.

The conversion of fat to CO2 and H2O happens during cell respiration. Cells prefer to use sugar for energy, but of course our bodies have processes to pull out and break down fat into molecules that can be used for energy in cells. These two usages happen at the same time, along with breaking down and burning proteins, but they happen at different rates.

We don't have control over the rate most of our cells work, but we do have control over our muscles. That's why we have to work out if we want to lose more weight than our basal metabolism will burn. When you're working out hard, you can lose 8-10 times as much CO2 as when being a couch potato.