What generates heat in the body during exercise?

You get hot and start sweating to cool down, but what physically generates that heat? Is it friction of blood moving through blood vessels? Is it some sort of chemical reaction happening?

Edit: Blown away by the responses! Thanks everyone so much for your informative answers, it really enlightened me more than I expected. I’ll think about diet and exercise in a different way from now on and gained a new respect for all the things my body does