I don’t know where I got this but I suddenly thought of it. Nice easy puzzle.
There’s a windowless room with 3 light bulbs hanging in random places but low enough for you to touch. All are off.
Outside the room there are 3 light switches, numbered 1 to 3, clearly marked with On/Off positions, all in the Off position.
You are told that every light switch switches on one of the lamps in the room, and if you can tell within 5 minutes which is which you win a prize. (Less friendly versions of this puzzle have you killed if you can’t, but I’m not that cruel.) You’re allowed to enter the room exactly once in those 5 minutes.
(A couple of people commented but all on Twitter. Solution now revealed.)
Switch on 1 and 2 and wait a minute. Then switch off 2, leaving 1 on. Go into the room. The lit bulb is 1, the unlit but hot bulb is 2, the unlit cold bulb is 3. Obviously, this won’t work any more in a couple of years when incandescent bulbs are completely obsolete.