If it's thick, like an orange, banana, grapefruit, etc. it's a peel and you peel it. If thin, like a grape, it's a skin. (There's an idiom---"by the skin of our teeth," which means "just barely," e.g. "We won the game by the skin of our teeth.") Potatoes are somewhere in the middle--restaurants serve "Potato skins" as an appetizer, but most people would say "Peel the potato." Hope this helps...