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I don't get the derision for these kinds of problems. And I think it's unlikely Feynman would have found this unhelpful. These are commonly referred to as Fermi problems. Fermi was a physicist of Feynman's generation - they were probably standing together at Trinity when Fermi did the famous yield approximation. The other physicists held this ability in such esteem they gave it a name and remembered him for it.

The question is meant to gauge whether you can create a simple model of a system that is completely foreign to you, but for which you are familiar with the fundamental rules governing it. The absurdity of it serves to remove bias. You don't have to worry about whether or not the candidate happens to have experience filling airplanes with ping pong balls. So much of effective programming is creating a model for the processes we are trying to automate. Coming up with effective abstractions uses many of the same skills.

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Indeed. Feynman talked in ‘surely you’re joking..’ about his affection for MIT students in his fraternity setting one another puzzles, and he talked about the question of how trains go round corners being one he found particularly interesting. In fact he repeats it in this interview: https://www.youtube.com/watch?v=y7h4OtFDnYE

He doesn’t answer the question ‘how does a train stay on the track’ with ‘some trains don’t’; he doesn’t switch to talking about rollercoaster trains and insist that the way they stay on the track is with horizontal wheels. He finds the logical, geometrically satisfying answer quite delightful.

And in fact, in many cases the flanges which he says are just for safety and aren’t supposed to impact the track because otherwise they make a terrible sound… are expected to hit the track to help the train make it round particularly tight turns. Coming from New York and having ridden the subways you’d think Feynman would know there are some corners subway trains take where the flanges rub all the way round, and the lovely conical wheels aren’t what keeps the train on the track at all.

So I definitely find this a weird characterization of a Feynman-ish approach to this kind of problem. I think he’d probably have delighted in the geometric neatness of why circular or triangular manhole covers can’t fall down their holes, and been happy to ignore the practical fact of square ones for the sake of a good puzzle story.

The 747 ping pong ball estimation problem is a Fermi problem, where the goal is to get the right order of magnitude of the answer. Sure.

But "why are manhole covers round?" is not. It's a stupid "have you ever heard this clever fact" question that's often designed just to figure out if you're clever and/or lucky vs testing actual knowledge and skill.

If your aim in interviewing is to fill the team with people who love being clever then it's a great technique. But in my view, it's better to interview for the skills the job requires.

I worked and interviewed a lot of people in business consulting. We did ask what you call Fermi problems as they are surprisingly representative of the abstract thinking that is required in the job, specifically proposing frameworks for what kind of information we need to either go and get or state as assumptions in order to support a decision. As the GP points out, the questions are testing how you model something you don't know but have common sense about, and significantly, how you don't get caught up in uncertainty or irrelevant minutia and identify some core assumptions.

That said, I agree 100% with you about "trick" questions. It seems some people cannot distinguish between "market sizing" (Fermi problems) where you need to make reasonable assumptions and describe a model of something, and "a man is found in scuba gear in the middle of the forest" brainteasers for kids. I have no idea why anyone asks brainteasers in an interview.

> I have no idea why anyone asks brainteasers in an interview.

I'll go one further - interviewers doing Myers Briggs Personality Tests.

This is really good insight and reminds me of similar industry norms that often get strong pushback (e.g. OKRs). Often there is solid reasoning behind why and when you'd want to employ these practices. However, enough organizations implement them just because they've seen them used or because there's hype around them. Eventually the poor implementation attracts pushback, but often the naysayers also don't understand the reasoning behind the practice.

I personally didn't understand what these types of questions would accomplish, but your explanation makes sense and it seems like this would be a good tool in the hiring toolkit.

I do agree that logic puzzles which require a single observation and/or have a binary outcome tend to be very bad interview questions. That being said, I think the original post demonstrates that the issue with the manhole cover problem isn't necessarily the question itself, but how it's evaluated. I'd be quite pleased if I were asking the question and the interviewee gave that answer (though I might raise the point that the configuration necessary to drop a rectangular cover is also one of the most ergonomic for holding it). The general notion that there is a single right and wrong answer is incongruous with a class of question used to evaluate the candidate's reasoning ability. Though I wouldn't be surprised if these were historically evaluated appropriately and candidates simply misinferred why they were being rejected.

> But "why are manhole covers round?" is not. It's a stupid "have you ever heard this clever fact" question that's often designed just to figure out if you're clever and/or lucky vs testing actual knowledge and skill.

I once had this interview question, gave them the expected answer, and cited the source of that answer. I never did get that job.

I am going to take your claim a step further and assert that this type of question can only reliably test cleverness if the interviewee is properly primed (e.g. there was a prior discussion of workplace safety), then the important thing is explaining why. Otherwise the only way to arrive at the answer is to realize that construction/maintenance workplaces place a high priority on safety. That isn't the type of thing you're likely to think about during an interview for a programming position.

An interesting little tid-bit: I've read that Feynman was sitting in a truck at Trinity, and I think he was alone. Feynman calculated that the truck's windscreen would absorb enough of the bomb's UV emissions to obviate the need for additional eye protection. The same source (which I cannot currently locate) said that everyone else was wearing welding masks/goggles.

Surely You're Joking, Mr. Feynman

Highly recommended.

If he was alone why was "everyone else" wearing goggles?

You really gunning for that QA job, are you ?

The question is meant to gauge whether you can create a simple model of a system that is completely foreign to you, but for which you are familiar with the fundamental rules governing it.

Then talk about a real problem, for chrissake.

You're forgetting that you're "interviewing" someone who not only cranked out supercritical calculations for the first atomic bombs - but the first working models for path integral calculus in QM, and so much other stuff.

And you're asking him about... ping pong balls and 747s?

Granted, your average dev doesn't get to do work quite as interesting as what Feynmann did. But still, if they're senior-level - and especially if they're at least reasonably good - then there's a 99 percent chance they can easily whip out 5 or 10 "end-to-end" problems that they've solved for which the guts were much, much meatier than those of...

Any of these jerk off problems that it used to be almost de riguer for interviewers to ask, a few years back.

Fortunately this practice is starting to fade from popularity, but unfortunately, not quite fast enough.

That would be true if everyone wasn't also asking the same questions. So you are just testing whether the person knows that you ask that question or has interviewed at other places that do (I have seen this in other industries too: firm adopts some "interview technique", no women get through, no under-represented groups get through, they are baffled but put it down to their process being a completely objective identifier of innate genius...kind of ignoring that some of these places are hiring tens of thousands of people who, by definition, aren't geniuses...people are weird).

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