There's a Feynman story in one of his books where he shows a university class a french curve (a very curvy plastic shape used in technical drawing) and explains

There's a Feynman story in one of his books where he shows a university class a french curve (a very curvy plastic shape used in technical drawing) and explains to them it has the amazing property that no matter how you turn it, the lowest point is tangent to the horizon. No one in the class realized he was just messing with them.