Here is a classic lateral thinking puzzle.
A very strange house has a single light bulb connected in the basement. The switch for this light is located on the top floor. Actually, there are three switches on the top floor: two of them do nothing, and the third controls the light in the basement. It is impossible to tell whether the light is on or off when you stand by the switches. How can you determine which switch controls the light bulb while only walking down to the basement one time?