There is a puzzle from the game Silent Hill 2 that I always tell people and it seems to confuse some although the answer is simple.
Basically you have a door you need to get through with a number keypad to unlock it. In a room is the clue on a black board that says "This months code is T, last month it was X and the month before that it was Z. But what are they going to do next month because that's all you can define with 4 numbers.
