A downstairs panel contains three on-off switches. One of them controls the lamp in your attic. Using only ONE trip up to the attic, how do you determine which switch controls the lamp?
Turn on the first switch and leave it on for a few minutes. Turn on the second switch and go to the attic. If the lamp is on, the second switch controls the lamp, if you touch the lamp and it is warm, the first switch controls the lamp, otherwise, the third switch does.