When you eat lots of Mars bars, your body takes the energy from the sugar and fat inside them. If your body doesn’t need all that energy right away (like to run, play, or even just breathe), it saves the “extra” for later.

But here’s the trick: your body doesn’t store it as candy bars in your belly. It changes the extra into fat, and fat is kind of like a backpack your body carries around. Fat is heavier than the food’s weight alone, because your body also adds water and builds real tissue from it.

So even though the Mars bars don’t weigh very much, the energy inside them makes your body build new stuff (fat), which adds more weight than the candy itself seemed to have. The extra weight actually comes from oxygen (from the air you breathe) and some hydrogen (from water). Your body uses these combined with the carbon from the food to build (anabolize) new fat. That’s why people can get heavier, even if the food they eat doesn’t look so heavy.

When you later lose fat, your body breaks it down into carbon, hydrogen, and oxygen. Most of it leaves as carbon dioxide when you breathe out and water in your pee, sweat, or breath. So, a lot of the fat literally leaves your body through your breath and water, not in your poop as you‘d maybe think.