Light bulb

Here is a classic lateral thinking puzzle.

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?

Help

Hint

Think about real lightbulbs, not mathematical abstractions.

Solution

Flip the first switch and wait 5 minutes. Then flip it off and flip on the second switch. Now walk down to the basement. If the light is on, then the second switch is the one that controls the light. If the light is off, you feel the bulb. If it is hot, then the first switch controls the light. If it is cold, the third switch (which you never touched) controls the light.