When you exercise, are you burning calories? Yes.

Are you literally burning calories, as through combustion the way most people understand it? No.

I guess a way to look at this would be akin to community trash fires that are sub-combustion temperature. Yay, now we don't have to truck it 100km to the incinerator, we can just burn it in the village square.

Composting is really challenging because a number of waste products, from citrus to inorganic or hybrid-inorganic garbage, can ruin the compost process and compost product. If someone leaves a metal nail in the compost pile and someone else later plants potatoes in it, that nail could end up in your mouth after a potato grows around it.