There are three switches downstairs. Each corresponds to one of the three light bulbs in the attic. You can turn the switches on and off and leave them in any position.
How would you identify which switch corresponds to which light bulb, if you are only allowed one trip upstairs?
Turn on 2 of the switches & leave it for a few minutes. Then turn off one of those two. When you go up the the attic, the one that's lit will be the one of the 2 that you didn't switch off, the one that's off but is warm will be the other one you had switched on but had then switched off. The one that is unlit and cold will be the one you never switched on.
