You are standing outside a closed door. On the other side of the door is a room that has three light bulbs in it. The room is completely sealed off from the outside. It has no windows and nothing can get in or out except through the door. On the outside of the room there are three light switches that control each of the respective light bulbs on the other side of the door.
Your assignment is to determine which light switch controls which light bulb. You are allowed to enter the room only once, and once you come out, you must be able to state with 100% certainty which light switch controls which light bulb.
Turn one light switch on, wait a few minutes, then turn it off and turn another light switch on. Go into the room and feel the light bulbs. The one that’s still warm is connected to the switch that you first turned on, the one that is on was the second switch you turned on, and the last bulb is controlled by the switch that you didn’t touch.
Turn two switches ON, and left one switch OFF. Wait 10 minutes. Then shut one of the ON switches OFF. One switch is ON and two are now OFF. Immediately go into the room. If the light is on, you know it’s the switch that is still ON. If the light is off, feel the light bulb. If the bulb is cold, it’s the light switch that you never turned ON. If the bulb is still warm but the light is off, it’s the switch you turned ON but then turned OFF prior to entering the room.
Switch on the first switch for 5 minutes and turn it off and then switch on the second one and enter the room. The lit bulb represents the switch you just turned on and then inspect the other bulbs by touching them, one shall be warm and the other shall be cold. The warm bulb belongs to the first switch you turned on and the cold bulb belongs to the switch you never turned on.
8 Comments on "Which Switch?"
Vanessa says
May 18, 2015 @ 17:16
I think this website has awesome riddles. They also have challenging ones.
Riddler says
December 18, 2015 @ 06:54
Part of the question is in the Show the Answer tab. Please fix it.
Dan says
December 18, 2015 @ 09:40
@Riddler Thanks for letting me know. It’s fixed.
The Sphinx says
December 18, 2015 @ 19:47
This is a really good riddle. Thanks for fixing it. When I read it, it didn’t make sense and I searched the answer. Heheh. I’m Riddler, btw.
Chloe says
January 31, 2016 @ 23:44
I agree there shouldn’t be a show answer key with the riddle
??
Abby says
August 9, 2016 @ 19:00
Turn two switches ON, and left one switch OFF. Wait 10 minutes. Then shut one of the ON switches OFF. One switch is ON and two are now OFF. Immediately go into the room. If the light is on, you know it’s the switch that is still ON. If the light is off, feel the light bulb. If the bulb is cold, it’s the light switch that you never turned ON. If the bulb is still warm but the light is off, it’s the switch you turned ON but then turned OFF prior to entering the room.
Dinesh Bhimsaria says
October 21, 2016 @ 03:16
Switch on the first switch for 5 minutes and turn it off and then switch on the second one and enter the room. The lit bulb represents the switch you just turned on and then inspect the other bulbs by touching them, one shall be warm and the other shall be cold. The warm bulb belongs to the first switch you turned on and the cold bulb belongs to the switch you never turned on.
guy says
October 13, 2018 @ 05:40
Can you believe I got this question ina job interview many many years ago? U got it right, and got the job. Tech interviews used to be so irrelevant.
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