You're in a house, and there is lamp upstairs that is off. In the basement are three switches. One of the switches turns on the lamp, the other two do nothing. From the basement there is no way to observe whether the lamp upstairs is on or off. You are allowed to flip two switches, after which you must be able to say with certainty which of the three switches turns the lamp on/off. What do you do?

Answer (select grey block with your mouse to view):

You flip the first switch on, wait 10 minutes, then flip it off again. Then you flip the second switch on and go upstairs. If the lamp is on the correct answer is the second switch. If the bulb is warm the correct answer is the first switch. Otherwise it's the third switch.


What logic riddles do you know?

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