A fact that is really really obvious, but only after having been pointed out.
Something that makes perfect, blatantly obvious sense, but only after giving it a bit of thought.
Bill: Did you know, when you smell a fart, you're actually smelling little particles of shit that get stuck up your nose.
Ben: Oh yeah! OMG I'd never thought of that before!
Bill: Obvious really; there's a nice bit of advanced common sense for you.