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Did You Find Out? Get the Answer to the Riddle of E-News Issue No. 2, April 2016


Which is the correct light switch?

How can you determine the right light switch?
How can you determine the right light switch?

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.

  • Yes, the lamp is on. The second light switch is the correct one.
  • No, the lamp is not on. If the lamp is still warm, then it is operated by the first light switch.
  • No, the lamp is not on. If the lamp is off and cold, the third light switch is the correct one.


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