The Singular Problem of "Skies" and "Brains"

Why do we say "skies" and "brains" when we only have one of each?by Dave SegalSolid album by the ex-Fleetwood Mac guitarist, but the title is problematic.
English speakers habitually use "skies" and "brains" when referring to singular entities. This doesn't make sense. Go outside and look up. See that? It's the sky. There's only one, yeah? It is indisputable... unless you're on some potent hallucinogens—in which case, hook a brother up. Yet we say "skies" all the time, not thinking how abs