• I stumbled across something like this on the internet.* No doubt you could find the answer that way, but that’s no fun.

You are in a room with three switches that each control a different [incandescent] light fixture in another room. You cannot see from the switch room into the lamp room. Your task is to determine which switches control which light fixtures, but you may only go into the room with the lights once. How do you determine which switch controls which light?

You do not need any equipment or magic to do this. Yes, you can reach the light bulbs when you enter the lamp room. No doubt someone will answer this in the comments. If not, I’ll post the answer on Monday.

* Here.

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• Yeah, this riddle has lost it’s pizazz when the assumptions of what a “light” is no longer holds.

• They still haven’t killed all the energy inefficiency in fluorescent lights but knowing they are incandescent makes it easier. Or I suppose more dangerous, depending on how fast you are.

Anyways…spoiler

Leave switch 1 on for 10 minutes.
Turn switch one off, turn switch 2 on, quickly walk to room.
The light that is off and hot is light 1. Off and cool, light 3. On, light 2.

• Colin and pkc beat me to the punch.

• Er… Where does it state that the lights start out in the ‘off’ position? Or that the switches are installed per code?

I think this is a better approach. Go in the switch room. Turn all the switches to the middle position. This assumes the old-fashioned type of switches, where there is a middle position which will turn off all current (if these are true toggle switches the problem is not solvable). Read the entire archive of TheIncidentalEconomist on your iPad. This should allow the lights to cool. Now (A) flip two switches up for 1 minute and then (B) down for 1 minute. This will warm up two (and only two) bulbs, regardless of which direction is ‘on’.

Finally (C) flip both of those two switches up, wait 3 minutes, then (D) flip one down again (call it switch 1, the other switch 2), run into the light room, and make the usual observations. Here’s how we can tell which switch is which:

Bulb really cold: The switch left in the disabled position

Bulb on and really hot: Switch 2. It was turned on in (C) and left on.

Bulb off and not as hot: Switch 1: It was turned on in (C) and turned off in (D).

Bulb on, and mild: Switch 1 again: It was turned on in (B), then turned off in (C), but turned on again in (D).

Bulb off and mild: Switch 2: It was turned on for a bit in (B), then turned off in (C) and left off.

Light which is cold and off: The switch left in the disabled position.

Light