Ah I don’t quite do that- but I could!
I grow little crystals that if you head them up they produce electricity.
Where the body heat comes in is that you are quite warm- warmer than the air anyway. I make little devices that you could strap to your arm and they would suck up some of that waste heat you are giving to the atmosphere. It absorbed it- converts it into useful electricity and hey presto- your charging your mobile while just walk about.
Well this is supposed to happen, it’s not perfect just yet- but I’ll get there!