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?
Show solution
Solution
Answer: use light and heat.
Turn on the first switch and leave it on for a while. Then turn it off. Turn on the second switch and enter the light-bulb room.
Upon entering, there are three possibilities:
if the light bulb is on, the correct switch is the second;
if the light bulb is off but hot, the correct switch is the first;
if the light bulb is off and cold, the correct switch is the third.
So you can Identify with certainty which of the three switches controls the bulb by entering it only once.