In the basement of Grandfather Jacob's house are three light switches. Only one of them turns on the lamp located on the first floor, but Grandpa Jacob has forgotten which one it is. Does he now have to make three trips up and down the basement stairs to discover the correct light switch? Or, is it possible to find out which of the three light switches turns on the lamp, and still make only one trip up the basement stairs?
Grandfather Jacob turns the first light switch on and waits about five minutes. Then he switches it off again, and turns on the second one. While the second light switch is on, he goes upstairs to see if the lamp is on.