You've just moved into a new house, and know that one of the three switches on the panel by the front door turns on the light in the loft, but which switch? You don't want to keep climbing the four flights of stairs to the loft, so how can you work out which switch turns on that light while only making one visit to the loft?
(You need to use something about the nature of light bulbs to help you.)
Let the three switches be A,B & C
Turn on switch A...
Wait for a while(until u think that the bulb gets hotter)
Now turn OFF A and Turn ON B...
Visit to the loft........
If the bulb glows then it is Switch B
else touch the bulb if it is hot then Switch A
else Switch C