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Aug 3 / kkrizka

Riddle of the Lights

A few days ago I’ve come across an interesting puzzle at the Physics Forum. The general answer given is the same one as what I though, but I think you should give it a try too before reading the rest of this post.

You’re in a room with three switches that open three light bulbs in a separate room. You don’t know which switch turns on which light bulb. How do you find out which switch goes to which light bulb by entering the other room only once? You don’t have any help, you can’t see the other room while you are in the room with the other switches or vice versa, you can’t make any device that extends your reach into the other room, etc.

Turn on switch #1 and #2. Then after a few minutes turn off switch #2. When you go into the room, you should see one bulb on and that is the one corresponding to switch #1. Then touch the remaining two bulbs, and the one that makes you pull your hand back in pain (because it’s hot) is connected to switch #2. The remaining one is associated with switch #3.


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  1. Blossom / Sep 18 2008

    Nice puzzle but I did not understand it properly. It is a bit confusing.

  2. dj / Sep 29 2008

    You turn on switch number one and leave it on for a minute or two, and then turn it off. Turn switch number two on and go in the other room. The light on is switch two, the light that is off and hot is switch one, and the light that is off and cool is switch three.

  3. Interesting puzzle! I would assume the way to find out which bulb goes to which switch is the heat of each bulb turned on for different periods of time.

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  4. Me / Oct 29 2008

    Since it was a physics thing i half expected the answer to be fffffffffffEEEEEEEEEEEEEEEEEERRRRRRRRRRRRRRRRRRRRRRRRMMMMMMMMMMMMMM, but that would be too obvious…

  5. Dan St.Andre / Oct 29 2008

    Has anyone been able to get the wifi LED working when the wireless network is enabled?

    The bluetooth LED works fine both from software and from the switch at the front of the keyboard.

    ~~~ Saint 0;-D

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