1) Three switches*
In a room, there is a light-bulb hanging down from the ceiling. Its door is closed, and from the corridor outside you can not tell if the light-bulb is burning or not.
In another room there are three switches, one of which controls the light-bulb – but you don’t know which one. The three switches are all in their ‘off’ position.The two rooms are several corridors apart. Assuming you don’t want to walk more then necessary, how often do you have to check the room with the light bulb in order to find out which of the three switches controls the light bulb?
Kees Krol recently reminded me of this problem, though he or someone else showed me the problem some time ago.
If you think you solved this puzzle, you can check your solution here