In one room, there are three switches. They control three lightbulbs in an adjoining room, but you don't know which switch controls which lightbulb. All switches are initially in the "off" position. You can't see the other room from where the switches are.
Your task is to find out which switch controls which lightbulb. You can manipulate the switches all you want, and then go ONCE into the adjoining room to inspect the lightbulbs.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)
