Three switches, a light bulb in another room, and only one chance to get in to see it. It seems like there's not enough information — until you realize that the light bulb can tell you more than one thing.
The three switches
Riddle statement
In a room there are three switches. In another room, which you can't see from the first, there is a light bulb connected to only one of them.
You can manipulate the switches however you want, but you can only enter the light-bulb room once. The light bulb is one that gets hot when left on for a while.
How do you determine with certainty which switch controls the light bulb?