Interview questions gone wrong

I have asked 100's of interview questions to people I have considered hiring through the years. It never ceases to amaze me how varied people's answers can be to questions. Answers I never considered. So when I ask questions, I listen to, and consider the answers, I don't mark them 'right' or 'wrong'. Rather than try to explain some real ones, there is an old joke (many variations on it, this is mine) that explains it well.

A man goes to Apple for an interview as a senior developer. The interviewer gets more and more frustrated as the interview goes along. Here is the last part of the interview:

Interviewer: I'm now coming to the part of the interview where we ask questions to look at your creative thinking ability. I'm not looking for a technically detailed answer to any of them, I' just want to hear your quick answers and the reasons for your answers.

The first question is: Why are manhole covers round?

Interviewee: The question is wrong in its assumption of fact. "Manhole covers" are not round. Some are square. It's true that there are some round ones, but others are square and rectangular.

Interviewer: But just considering the round ones, why are they round?

Interviewee: If we are just considering the round ones then you are excluding all that are not round. So the set in question are round by definition. That statement is a tautology.

Interviewer: Let me try again, why are there round ones at all? Is there some particular purpose in choosing to have round ones?

Interviewee: Yes of course. Round covers are almost always used when the hole they are covering up is also round. It's easiest to cover a round hole with a round cover, it is a lot more work to cover a round hole with a square cover.

Interviewer: Um, can you think of a property of round covers that gives them an advantage over square ones?

Interviewee: Since the shape of the cover I am asserting is most likely determined by the shape of the hole they are covering, we have to look at what is under the cover to answer that question. The hole below the cover is round because a cylinder is the strongest shape against the compression of the earth around it, it would cost a lot more to have a triangular or square or rectangular hole liner that has the same strength against the ground compression. Also, the term "manhole" implies a passage big enough for a man, and a human being climbing down a ladder is roughly circular in cross-section, close enough that it wouldn't be worth going to an egg or oval shaped cylinder. So a cylindrical pipe is the natural shape for manholes. That covers the hard part of the question, and we easily work back that the covers are simply the shape needed to cover up a cylinder.

Interviewer: Let me try asking the question from a different angle: Could there be a safety issue? Couldn't square covers fall into the hole you are talking about and hurt someone?

Interviewee: The holes I have been talking about are round. If the hole they are covering is round, then there is not the slightest risk of the cover falling into the hole. A square cover in both width and length would be larger than the diameter of the round hole it is covering and therefore impossible to fall into the hole unless the cover is weak enough to be be bent, and then it doesn't matter whether it is round or square or any other shape.

Interviewer: I mean, what if the hole was also square? Wouldn't there be a safety risk there?

Interviewee: Not likely. Square covers are indeed used on prefabricated vaults where the access passage is also square, noting that in a vault, there is no need to worry about the compression strength of the earth, and so instead you can go to the more easily manufactured, and lower cost square passage. The cover is larger than the passage, typically 3" wider. The cover then sits on a ledge that supports it along the entire perimeter. The covers are usually made of solid metal and are very heavy. I'm sure you can do the simple geometric math required to see how unlikely this risk is. Would you agree that a two foot opening is reasonable?

Interviewer: (exasperated) Yes

Interviewee: In order for the square cover to fall in, a two-foot square opening and a inset ledge width of 1-1/2 inches, it would require, that you lift one side of the cover, then rotate, let's see, yes, about 30 degrees so that the cover could clear the ledge. Clearly a foolish idea that would take a lot of conscious thought and extra labor with no value, but even so, that is still not enough to fall in. Now you would have to lift the cover up nearly 45 degrees from horizontal before the center of gravity would shift enough for it to fall in. So yes, it's possible, but very unlikely. The people who are allowed to open manhole covers could easily be trained to do it safely.

Interviewer: (trying to gain control of the conversation) But what if a person passing by were to do so maliciously, or the second person on a two person team after the first person has climbed in?

Interviewee: So you are talking about a random person walking by who is attempting to commit murder or a person on the team? Given that I suspect most people who open a manhole cover use a tool like a hammer or a crowbar to lift the cover, it would be a lot less work for the person trying to commit murder to pick up the tool and throw or drop it down the hole on the person in the hole, and think about how easy that would be to do and get away with compared to noisily moving a manhole cover to drop it down the hole. As far as a second person on the team, if they went through the effort of rotating it 30 degrees and then lifting it at least 45 degrees, that would make a pretty easy case for the prosecution.

Interviewer (excitedly): Excuse me a moment; I have to discuss something with my management team. (Leaves the interview room.)

(Interviewer returns after several minutes)

Interviewer: We have several products that are not nearly as good as our competitors and we charge twice as much as theirs. We would therefore like to offer you a job, starting immediately, in the marketing department.