Riddle: a man lives in a house with no pets or family. He has 3 switches downstairs which lead to 3 bulbs upstairs. He needs to know which switch leads to which buld and he can only go up the stairs once. How will he find out which bulbs leeds to which switch?
Answer: He turns one switch on for half an hour then he turns it off then he turns another switch on and goes upstairs. Then he checks which bulb is hot and which bulb is on.