The three switches riddle

Here’s another nicely fiendish riddle.

There’s a light bulb inside a room. Three switches are outside the room, but only one turns the bulb on. The room has no windows and you can’t see inside from outside. Your task is to figure out which switch turns on the bulb. You can fiddle with the switches as much as you like, but you get only one look inside the room. After that, you have to identify the switch.

More riddles

• The marbles riddle
• The two mathematicians riddle