Post:
You have three switches in one room and a single light bulb in another room. You are allowed to visit the room with the light bulb only once. How do you figure out which switch controls the bulb? Write your answer in the comments before looking at other answers.
Comment:
If this were an interview question, the correct response would be "Do you have any relevant questions for me? Because have a long list of things that more deserving of my precious time than to think about this!
What bothers me about this specific question, apart from it being dated, is that it breaks the rules of these kind of riddles. They’re implied to be in a sort of frictionless sphere universe, the whole preposition is silly except as an abstract puzzle. To then rely on the physical properties of real lamps is cheating. You’re supposed to ignore all the real-world aspects of the setting except that one.
Agreed, it presents as an abstract logic puzzle, but then gives a very concrete answer. It’s like presenting the trolly problem to someone, and when they give one of the two expected answers saying “no, stupid, you run ahead and untie the victims before the trolly reaches them.”
It’s compounded by the fact that the proposed physical solution isn’t even very reliable, as lots of people in this thread have said. If we’re stepping outside of the logic puzzle constraints, why not just leave the door to the room open? Or have someone stand inside and shout when the light turns on? Or ask someone who knows these switches? Or any number of boring non-brain teaser solutions.
Remove the switches put a microcontroller like esp32, connected via wifi to an app on your phone. Go to the other room and see which switch switches on the bulb.
If there is no wifi, why the hell do you want a programmer. I can’t work without internet.
It depends on what type of person designed the circuit and what type of person you are.
Ergonomics: The switch closest to the door first, then mid, then far, figuring the unknown user would click the switch closest, a skilled electrician would start there. However, it’s not unreasonable for the electrician to ask the owner, so this is a hit-or-miss approach.
Installation efficiency: The installer refused to mark any of the lines and instead hooked them up at random, flip in any order, when you find the right one, return the others to the original state.
time efficiency: the energy cost to flip all three switches is minimal and you’re only going in once, flip all three at the same time. you’ve done maximum effort and maximum time savings.
Error reduction, binary counter, all combinations tested in case of chained switching
Debugging: binary counter, followed by checking the lightbulb, possibly swapping for another if one is nearby, checking all the other switches near the room, breakers, power to the structure, and asking an occupant for assistance as a last resort.
Disaster recovery: locate a flashlight or use your phone’s torch/flashlight function.
Ahh crap, other room.
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ask an occupant
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shove a penny in the socket behind the light bulb and listen for a breaker to pop
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turn all three on
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slide your cell phone under the door with video recording on, stomp on the floor hard every time you flip a switch
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turn all the switches through a binary counter looking for one that seems to do nothing.
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Unlabelled switches controlling lights in another room isn’t Workplace Health and Safety approved.
Lockout both rooms and log a job with maintenance.
Go into the room and unscrew the bulb. You can now truthfully say that no switch affects the bulb’s condition, without messing with a bunch of switches whose function you don’t understand. You even know for a fact that the lack of bulb won’t cause a problem down the line, since the room is apparently no longer accessible.
This question becomes more a test of age as time goes. I’ve been asked this question even after the movement towards all-LEDs.
This question is also stupid, both because it has a correct question and because almost certainly some people have advantages over others that have nothing to do with the actual job.
20+ years ago? Sure, this was a somewhat viable question. But now? It’s incredibly messy.
Over my years, I’ve asked dozens of very, very smart people from all kinds of walks of life, extremely smart to seemingly dumb as hell - nobody has ever gotten it right.
Probably the only thing this question is good for is seeing how an applicant does when faced with a diplomatic situation and a really dumb interviewer.
I’m super curious what the people who unironically ask this question think they’re testing.
It’s a silly riddle that, for some reason, has stuck around in my head for decades, I think from an old tv show (anyone else remember Crashbox?). I remembered the answer immediately. So, this would be less of a test of my reasoning/problem solving skills, and more of a test of my ability to find and store vast amounts of useless trivia and instantly recall it decades after the fact. If that’s what you’re hiring for, I’m your guy!
The official answer to this riddle is turn switch 1 on for a minute or so, switch it off then switch 2 on. if the bulb is hot but dark, its 1, if it’s lit it’s 2 and if it’s out and cold its 3.
the adult answer is why do I have only one chance to walk in the room?
this is the classic answer but it also fails pure logic because the question only implies one of them actually works, and even then, it’s only one of them. the truth is any number of them could work, or a specific combination, or a number of combinations, or it might be none. the bulb itself to could be busted. my point is not to be an uncooperative asshole but that a logic puzzle that relies on real world properties should cover its bases.
the adult answer is why do I have only one chance to walk in the room?
The actual adult answer is questioning why the switch is in a different room and if it’s because of safety, demand for safety protocol
if the bulb is hot
if hot they’re using out of date lighting, who the fuck uses incandescent bulbs this far into the 21st century? they have failed their interview with me.
LED do not have a 100% efficiency, and do produce waste heat. A lot less than an incandescence one, sure, but enough for that answer to be valid.
Well, maybe you’d better wait 10min instead of one, to make sure the led lightbulb heats enough, but still…note the premise specifies HOT.
none of my LED bulbs get hot even after hours. they do warm up from ‘cold’ but HOT?
ymmv.
Well, maybe you’d better wait 10min instead of one, to make sure the led lightbulb heats enough, but still…
I tested this with a 5W IKEA LED light-bulb, since I was just doom scrolling, anyway:
- After 1 minute of being on, the bulb was still room temperature.
- After 10 minutes of being on, the bulb was lukewarm.
- After 10 minutes of being off, the bulb was room temperature, though the fitting maybe felt slightly warmer. That latter will probably depend on your installation, and how well it is able to disperse the heat.
This means that the solution either breaks down entirely, or is unreliable, since you are not (reliably) able to tell the first two buttons apart
but enough for that answer to be valid
Highly arguable. Especially without specifications on the lamp. It could be a rather dim and small one. Then, you either need special equipment or supersenses.
The image does depict an incandescent filament bulb.
The question is outdated as fuck too. It’s not a new riddle.
yeah silly games for bored hiring managers
So I can’t go to the other room to set up a camera?
What if it’s a LED bulb?
LED bulbs do get warm, not as hot as incandescent bulbs but they do emit heat. You might have to run them longer than a minute to warm it up enough to be immediate about it.
“First, I would get a label maker and ask a coworker to assist me. Then, we’d work together to quickly figure out what each switch does, and then label them accordingly. In a business of this size and reputation, documenting your work and synergistic teamwork are foundational to value and growth.”
Then, reject whatever offer they send and say that it’s because they showed you a workplace culture that enabled middle management to test employees with busywork instead of minding their own business or solving their own damn trivial problems.
Here’s my answer that works with any kind of lightbulb.
Flip switch 1 on, switch 2 off, and get switch 3 stuck in a halfway point which I’ve done on both lever switches and flat switches.
If it’s on it’s switch 1, if it’s off it’s switch 2, if it’s flickering or dimmed it’s switch 3 and you should probably turn it off to stop damaging the relay.
Trying to get a switch stuck half way sounds like a good way to start a fire. If the bulb is dimmed, that means not all the power is making it to the bulb, and half of it is probably going into heating up the switch contacts. It could also be arcing inside the switch, which will also destroy the contacts. I think some new building codes require “arc fault protection” on circuits for this type of reason, in addition to “ground fault protection” (GFCI) on bathroom/kitchen circuits.
Ok. The classic answer is “turn on the first switch for five minutes. Then turn switch 1 back off, turn on the second switch and go in the room immediately. If the light is hot, it’s controlled by switch 1; if it’s on, it’s controlled by switch 2; if it’s off and cold it’s controlled by switch 3.”
Except that a light bulb in 2025 is very likely to be an LED bulb, so it wouldn’t actually get hot. At least not hot enough to feel even a few moments later. And in a corporate setting (this is classically an interview question), the switch has been more likely to control a fluorescent tube, which can get hot, but typically not as quickly as an incandescent one.
My answer, if I were in an interview, would be to ask questions (Chesterton’s Fence).
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First of all, why do we have the one-visit limit? Is this a prod light bulb? We need a dev light bulb environment, with the bulbs and switches in the same room. (While we’re making new environments, let’s get a QA and regression environment, too. Maybe a fallback environment, depending on SLAs.)
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Second, what might the other switches do? What’s the downside to just turning them all on? If that’s not known, why not? What is the risk? For that matter, do we know that only one switch needs to be turned on to turn on the light, or is it possible that the switches represent some sort of 3-bit binary encoding?
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Third, why were the switches designed this way? Can they be redesigned to provide better feedback? Or simplified to a single switch? If not, better documentation (labeling) is a must.
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Fourth, we need to reduce the length of the feedback loop. A five minute test and then physically going to touch the bulb is way too long. Let’s look into moving the switches or the light in our dev environment so that the light can be seen from the switches.
“why was I not equipped with current detectors as that is standard practice in the industry?”
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I would say, I do enjoy riddles, so this will be fun. But I am concerned that if you think my skill at riddles is critical, that it may mean your management has gotten used to not fully thinking through the objectives they give and how those objectives interact with the existing systems or other objectives. That would result in the kind of product that looks like the right hand doesn’t know what the left is doing. If that is your reasoning for the question, how is the company countering it to create a coherent product.
And the reason I might say this is tgat in my experience, companies who ask such questions aren’t the kind I want to work for.
Ha! Easy! Go in the other room and take a picture of the bulb. Now go back to the switches and flip each one in order, while looking at the picture. When the picture of the bulb shows it lit up, that’s the switch.
I’ll look through the door.
Or, set up a webcam to see when the light is on.
If this isn’t allowed somehow, I’ll tell the building management to consider rewiring this absolutely cursed light switch situation ASAP because it’s gotten so bad that it’s being used as a brainteaser by the recruiting department
Literally ask someone which switch it is. Then ask them what idiot wired them up that way
For those that want the actual answer:
Tap for spoiler
You turn on the first switch for a minute or two, turn it off, and turn on the second switch. If the bulb is on, it’s obviously the second switch. If the bulb is off and warm, it’s the first switch. If it’s cold, it’s the third switch.
This assumes several things to be true, which might not be true:
- power is available/the upstream circuit is on (always a bad assumption to make)
- the bulb is an incandescent type that will generate an appreciable amount of heat in a short amount of time
- the bulb was in the off state before you changed the position of any switches, and has been off long enough to be cold
- the bulb is connected to any of the switches
- the bulb is connected to only one of the switches (parallel circuits are a thing, as are multi-switch lighting circuits)
If any of the above is not true, the conclusion is invalid.
I’ll go one further:
- Assumes the bulb is in reach. When I read the problem I assumed the bulb was in a ceiling fixture out of reach. Nowhere in the text description did it specify the physical location, except “in the other room”.
The biggest flaw is that it assumes you’ll add conditions you’re not explicitly told are allowed. Many, many problems in school would be trivial if changing the terms beyond what’s stated was allowed.
This is often exactly what the interview question is testing. Many of these questions are not about the solution but about how the applicant approaches problems
Yet they never explicitly state you’re allowed to make convenient assumptions. If the bulb was out of hand’s reach the problem would be unsolvable.
Assuming the electrician that wired the switches is in the room would be even a more out-of-the-box solution.
As I said, they care about how you think. Do you ask all these questions?
if I were given this interview question I would immediately start asking questions: Do I have my phone? Can I bring any objects into the room? Do I know the construction of the light? How far from the room is the light switch panel?
Asking “what are the limitations and conditions of this situation” is literally the thing they want to see. That’s my entire point.
Also the image shows all 3 switches are on.
If I asked this question during an interview and the candidate gave me this list of assumptions, I would recommend the candidate. This is exactly what I would be looking for by asking a vague question, not if they memorized the answer to a bunch of riddles, but how they thought and what their line of thought was for troubleshooting the answer.
I tend to agree with this line of thinking. If you’re trying to hire an effective problem solver, well the first step to solving any problem is understanding the problem - the whole problem - and often more importantly the context in which the problem exists.
And while my first reaction is to be frustrated with the person asking for a solution to such a vague problem… in the real world problems are rarely clearly stated, and frequently misstated. Investigating the apparent conditions of the problem is always necessary, and generally the fastest path to resolution.
Also that the labels are as shown. For all we know the internal wiring is switched, and if that were the case then some could have Up=On while others have Up=Off but not all matching.
I love the idea of someone trying this stupid question irl only to realize it wasn’t even plugged in. That’s … well fuck, that’s most IT work. The convoluted approach is definitely the wrong one. Lol
Also:
- I still remember which switch is which after having checked the bulb
You’d be boned if it’s an LED bulb that doesn’t warm up noticeably.
This riddle has been around for so long it predates LEDs and pedantic software engineers.
Or if it was turned on to begin with and you just turned it off
Text ambiguous. Leave doors(s) between rooms open. Flip switches, see which one controls bulb in other room. No need to even visit other room. Done in seconds.
Don’t even need to leave the door open. What door doesn’t have enough of a gap to see if a light is on?
This also assumes youre alone, a practical person would send someone else in the other room and communicate the states back
It doesn’t assume you’re alone.
tap for comment to spoiler
Nice try, they recently upgraded to led lights.
Assumes that the bulb can be touched, that it is hot when turned on, and that the position of the switch for ‘on’ is the standard position.
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