You are facing three switches downstairs, one of which controls a lightbulb in a room upstairs whose doors are closed (no light escapes from it).
Find the correct switch without ever going down the stairs.
Turn on #1 and #2 only. Wait... Turn off #2. Rush upstairs. The correct switch is #1 if the bulb is lit, otherwise, it's #2 if the bulb is warm, or #3 if the bulb is cold.
