On a related note, the funniest puzzle I ever heard of was one of those classic "One of us answers all questions truthfully, the other always answers in lies. Behind one lies certain death, behind the other, the way deeper toward the treasure you seek. You may ask only one question, and ask only one of us, if you wish to pass."
Thing is, the PCs come to a room in a dungeon where there are two giant skulls with long, jagged teeth, sitting over two doorways. The way they're placed, if you tried to walk through either door, the skull could easily chomp down on you and tear you to bits.
The eyes of the skulls flare with fire, and the PCs listen as the skulls say their riddle. They spend a few minutes discussing what to ask, and eventually hit on a proper question, "Would the other skull answer that its door is safe?"
Option A: Truth door, deadly way. He answers "No," because the other, lying door would not be honest that its door is safe.
Option B: Truth door, safe way. He answers "Yes."
Option C: Lying door, deadly way. He answers "No," because the other, truthful door would say its door is safe, but this door is lying.
Option D: Lying door, safe way. He answers "Yes," because the truth door would tell you its own way is unsafe, but this door is lying about that.
So if the door you ask answers Yes, you go into that door. If the door answers No, you pick the other door. Simple, right?
Well, yeah, except why the hell are you listening to a pair of giant talking skulls that could chew you to pieces? The PCs asked their question, got an answer, and chose one of the doors, but of course the skulls were just lying either way, trying to lure the poor bastards into their mouths. I'm serious, people become stupid when they're asked to be smart.