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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?
what is keeping the first bulb on, turn on the second switch for a bit so it gets warm, and don't turn on the third at all. The bulb that is on is the first switch, to tell which is the second switch touch the bulbs and see if its warm and the one that is off is the third.