Three Bulbs and Three Switches
The puzzle
Outside a windowless room there are three ordinary light switches. Inside the room there are three ordinary bulbs, one controlled by each switch. The door is shut and you cannot see in.
You may flick the switches as much as you like while the door is closed. You may open the door and go in exactly once, and after that you may not touch the switches again. Which bulb belongs to which switch?
Stuck?
One at a time, smallest first. Open only as many as you need — the whole point of the puzzle is the part you work out yourself.
Hint 1
A bulb that is off tells you nothing - unless it is off in two distinguishable ways.
Hint 2
What else does a bulb do besides emit light?
Show the answer Hide the answer
Turn on the first switch, leave it for ten minutes, turn it off, turn on the second, then go in. The lit bulb is the second switch, the warm one is the first, and the cold one is the third.
How to get there
Turn switch 1 on and leave it alone for about ten minutes - long enough for its bulb to get hot. Then turn switch 1 off, turn switch 2 on, and immediately open the door.
The bulb that is lit is switch 2, because that is the only one currently receiving power. Of the two dark bulbs, touch them: the warm one ran for ten minutes and so belongs to switch 1, and the cold one never ran at all, so it belongs to switch 3.
The technique worth keeping
One trip into the room looks like one bit of information per bulb - lit or not - and one bit cannot separate three cases. Heat is a second bit, and it is sitting there unused. When a puzzle seems to be one measurement short, the question to ask is what else the thing in front of you is doing besides the obvious.