There is a hallway, and on both sides of the hallway there is a door. Behind each door is a room. Both rooms are completely empty. In the one room there are just three lightbulbs hanging from the ceiling. They are not burning. In the other room, there are three switches. Only one switch will turn on a lightbulb in the other room, but you don't know which switch. You are allowed to turn on as many switches as you like, but you may only go once and have a look in the other room to find out if there is a lightbulb burning.

How can you be absolutely sure which switch turns on the only connected lightbulb?

Solution: Turn on the first switch and do nothing; just wait for half an hour. Then turn the first switch off and turn the second one on. Now you walk to the other room. If there is a lightbulb burning, that's because of the second switch. If there is no lightbulb burning, go and feel the lightbulbs. If one lightbulb is warm, that is because of the first switch you had turned on. If all the lightbulbs are cold, you know for sure that the third switch will turn on a lightbulb.