How does fat get out of your body, after you've used it as fuel? Lots of ways, including all those "excretions" people usually talk about. But.... 80% of it.... is exhaled. http://www.bbc.com/news/health-30494009
Now that doesn't mean just breathing makes you lose weight--this is just how the fat gets out, after you've used it up by expending more calories than you take in.
Still, it helps me understand why, back when I was walking an hour a day or more, it didn't seem to help weight loss at all--but when I started doing 20 minute stints of kickboxing, suddenly weight seemed to melt off. I wasn't walking fast enough to work up to huffing and puffing. Kickboxing gets me exhaling like crazy.
Kickboxing stays in my tool kit!