There are two rooms separating by a wall, one room has three lamps, the one one has three switches, can you tell how to find out which switch is for which lamp if you only allow to look at other room once. (Question from Bill Gates, he said if anyone can answer this question in five minutes, he would hire that person.) So far I asked many people this question, only one guy could answer this question within 5 minutes, he was a geek though.