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The Three Light Switches


You are outside a room facing three switches. Inside the room is a light bulb. One of the switches is genuine, two are dummies. All three are in the off position, as is the light.

You are allowed to enter the room only once and once inside the room you cannot leave it. How do you work out which switch is genuine?

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How can you tell if a light has been on?

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When you enter the room, you have to touch the light bulb.

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Turn on one switch and wait five minutes. Then turn that switch off and turn on another. Enter the room and touch the light bulb. Obviously, If the light is on the genuine switch is the second one. If the light bulb is off and cold it is the third one and if the light bulb is warm and off it is the first switch. If the light had been turned on for five minutes it would have warmed up.

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