Turn on a lightbulb - Solution

You turn two switches ‘on’ and one switch ‘off’ and you wait…

Just before entering the room you turn one switch from ‘on’ to ‘off’.
If the lightbulb is warm, but off, it has to be that last switch.
If it is on, it has to be the switch that is turned on.
If it cold and is off, it has to be the switch which was already off.

Peter Hosszu has sent an excellent update on this riddle:

‘I think you could have four switches (let’s name them A, B, C and D) instead of three.
The solution in this case:

Turn on A and B for a while, then turn off A and turn on C.
The switch in control depending on the state of the bulb:
A warm and dark
B warm and light
C cold and light
D cold and dark.’

Maybe now you could help these ladies with their light.

12 Responses to “ Turn on a lightbulb - Solution ”

  1. Jerid
    October 1st, 2007 | 6:31 pm

    I’d just turn them all on

  2. Mysterymouse
    November 7th, 2007 | 7:03 pm

    I’d just turn them all on I mean the lightbulb will still be on right?

  3. Andy
    November 15th, 2007 | 2:53 am

    its so easy to do this …. the easiest solution is ………. USE A TORCH !

  4. John
    December 4th, 2007 | 9:54 pm

    Something about the answer is incorrect.

    “You turn two switches ‘on’ and one switch ‘off’ and you wait…”

    This defeats the conclusion.

    You turn ONE switch on and wait.

    Before entering the room turn it off and turn another one on.

    Therefore if switches are A, B and C:

    A on
    B off
    C off

    Before entering the room:

    A off
    B off
    C on

    If the bulb is warm (off): A
    If the bulb is cold (off): B
    If the bulb is on: C

  5. tessha
    April 17th, 2008 | 11:17 pm

    but what if the light was on to begin with…. then it doesn’t work.

  6. Roni
    May 3rd, 2008 | 3:11 pm

    my head hurts just thinking about it :P

  7. nobody
    May 16th, 2008 | 9:25 pm

    mira cuando se ponen acertijos complejos no deves de tomar las cosas externas a lo dicho en el mensajeasi que yo creo esta mal empleada la pregunta mejor les pondre uno muy sencillo:
    el primer numero es mayor que 2°
    el segundo numero es el doble que el tercero
    el tercer numero es uno menor al cuarto
    el cuarto numero es la mitad del primero

    si desean las dos soluciones posibles solo contactenme

  8. lauren
    June 10th, 2008 | 7:12 am

    WoW this is sooo confusing
    why dont you just turn on one light switch and walk into the room and see if its it if not try oher lightswitches
    THIS IS NOT HARD PEOPLE!!

  9. Liz
    August 7th, 2008 | 2:32 pm

    Not that the riddle stipulates, but you could do it in fewer flicks…

    Turn 1 on for five minutes
    Turn it off and 2 on
    go in the room…
    if light is on, it’s 2, if it’s off and cold, it’s 3, if it’s off and warm, it’s 1.

    Just going for efficiency and saving electricity.

  10. sarita
    August 17th, 2008 | 6:04 pm

    Or…. you walk into the room and there’s a lightswitch on the inside, which is the one that controls the light. Then you’d definitely know.

    That was my guess.

  11. Alfie
    December 27th, 2008 | 10:26 am

    Why can you only enter the room once? Just look round the door :] This is a silly question ^,^

  12. ruth
    February 15th, 2010 | 8:06 am

    it’s not a “do watevers easiest” its a riddle you have to work it out the clues in the name “puzzle”

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