You are in a room with three switches, labeled $A$, $B$, and $C$. Initially, each switch is in an off state. In an adjoining room, there are three incandescent light bulbs. Your task is to correctly label which switch goes to which bulb.
The catch: you may only enter the bulb room once. You have as long as you need in the switch room, and you may flip any switch as many times as you'd like, but once you leave the switch room, you may not enter it again.
There is a way to know for certain which bulb goes to which switch. How?