There's one that always boggles me, from the movie Labyrinth as most popular but I see it everywhere else.

Two gargoyles, one speaks the truth and the other one lies. You are to use only one question to get a clue as to who to believe (the one not lying). The person asks, "Are you lying?" and from there I get lost because both the gargoyles say "No." Or maybe I'm getting this really wrong.