Picture yourself in a room with three switches on the wall and a closed door leading to another room. Each switch turns on one light bulb in the next room, but you don’t know which switch works which bulb. Here’s the tricky part: you can flip the switches as often as you like while in your current room, but once you open the door and step into the other room, you can’t come back to mess with the switches again. How can you figure out which switch controls each light bulb using one trip to the other room?
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Solution:
Turn on the first switch and leave it on. Switch on the second one and let it run for about five minutes then turn it off. Don’t touch the third switch at all. Now head to the other room.
This is how you can tell which switch goes with each bulb:
The lit bulb matches up with the first switch (which stayed on)
Of the two dark bulbs, the one that’s warm when you touch it is linked to the second switch (which was on long enough to heat up)
The bulb that’s still cool when you touch it is connected to the third switch (which never got turned on)