You are outside a room with 3 switches on the wall.
One of the switches will turn on a light bulb in the room. The other two have no effect.
You can switch the switches as often as you like in any combination but only one of them will turn the light bulb on.
You can only enter the room once.
So, how can you tell which switch turns on the light bulb?
There are no tricks involved.
Blm
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At first glance you might think you can only detect one thing with the light bulb – ie it is off or on.
There is however the heat factor… Once you twig that a bulb heats up when its on then the answer is just a case of working out what to do and check for.
…
Put switch number 1 on for 30 seconds or so to give the bulb time to heat up
Put switch 1 off and put switch 2 on.
Enter the room.
If the bulb is on then switch 2 controls it
If the bulb is off and cold then switch 3 controls it
if the bulb is off and warm then switch 1 controls it.