Job Search Tip of the Week #11 (2018)


How to Ace Your First-Round (or Really, Just About Any) Interview

Think Like an Interviewer – Q5

Contrary to what the above cartoon suggests, interviewers don’t ask “Why are manhole covers round?” to trip you up – there’s method in their madness, so to speak.

“Why are manhole covers round?” became a popular interview question ever since Microsoft reportedly began using it two decades ago in order to identify the job candidates who think outside the box.

That being said, interviewers are looking for more than just creativity. “Why are manhole covers round?” and other lateral thinking interview questions are meant to also reveal your thinking process, most specifically your deductive reasoning skills.

So sadly, “Manhole covers are round because if they weren’t the Teenage Mutant Ninja Turtles couldn’t spin really fast and fit down the hole to go home” (one of our favorite answers to this question, said in all seriousness by a five-year-old), probably wouldn’t land you that second interview. Because although creative, it lacks logic.

Thus, when responding to “Why are manhole covers round” and questions of the like, it’s important to remember that you’re being evaluated more for how you approached the problem and arrived at your answer, than the answer itself.

In other words: Are you willing to entertain a “silly” question with seriousness? Can you look at a problem from many angles? Can you think logically? Is there a method to which you consider, accept, or reject plausible solutions before proposing one?

Engaging in deductive reasoning, or “top-down” logic, will get you there. It’s the process in which a specific, logical conclusion is derived from one or more general premises that are assumed to be true. There are three forms of deductive reasoning:

  • Law of detachment. Better known as the if-then statement: If P is satisfied, then Q exists.
    Basic Form: P –> Q (conditional statement). P (hypothesis stated). Q (conclusion deduced).
    Example: If the manhole is round, then the manhole cover is round (P –> Q). The manhole is round (P). There, the manhole cover is round (Q).
  • Law of syllogism. Or as we like to call it, “guilty by association.” It’s the forming of a conclusion by combining the hypothesis (P) of the first conditional statement with the conclusion of the second conditional statement (R).
    Basic Form: P –> Q. Q –> R. Therefore, P –> R. (You mathematicians will draw similarity to the transitive property of equality, where A=B, B=C, therefore A=C.)
    Example: If you’re in the United States, then the manholes will be round (P –> Q). If the manholes are round, then the manhole covers will be round (Q –> R). Therefore, if you’re in the United States, then the manhole covers will be round (P –> R).
  • Law of contrapositive. Given a conditional statement, if the conclusion is false, the hypothesis must be false also. Think of it as working backwards!
    Basic Form: P –> Q. ~Q. Therefore, we can conclude ~P.
    Example: If the manhole is round, then the manhole cover is round (P –> Q). The manhole is not round (~Q). Therefore, the manhole cover is not round (~P).

Deductive arguments are evaluated for their validity and their soundness. You have validity if you affirm the premises and the conclusion, or deny the premises and the conclusion. In other words, you cannot ever have true premises and a false conclusion, or vise versa. You have soundness if your argument is valid and your premises are in reality true.

For example, take the example we provided for you in the “Law of syllogism” section: If you’re in the United States, then the manholes will be round (P –> Q). If the manholes are round, then the manhole covers will be round (Q –> R). Therefore, if you’re in the United States, then the manhole covers will be round (P –> R). This is a valid but not sound argument, as you can find triangular manhole covers in Nashua, NH (pointing in the direction of the underlying water or sewage flow), and also in San Francisco, CA and Hamilton, Bermuda. There are also square or rectangle manhole covers. So because the first premise (P –> Q) is in reality false, it cannot be sound. However, it is a valid argument if you assume both premises to be true, or don’t know any better to say otherwise.

Here are the most commonly proposed explanations1 (We strongly encourage you to flex your creative muscle and either expand upon the ideas presented or refute them, rather than simply regurgitating them in an interview.):

  • If you’re considering only the round manhole covers, they are round by definition. So they are round because they are round. If you’re considering the value of a round manhole over any other shape…
  • Circles are more aesthetically pleasing in-between the straight lines and edges of the road.
  • A round manhole cover can’t fall through a hole that’s smaller than it, no matter what angle you turn it (although, a manhole cover in the shape of a Reuleaux triangle or any other polygon of constant width would also serve this purpose).
  • Round manhole covers don’t need to be rotated to align properly with the shape of the manhole (if cut precisely), unlike triangles, squares, or rectangles.
  • You can lock a manhole cover into place simply by turning it if 1) it’s round and 2) if there’s a notch. A shape with edges gets dislodged more easily when pressure is applied to one of its corners, so you’d need to bolt it down or make the cover very heavy so that traffic passing over them doesn’t lift it up by suction. A round manhole cover thus uses less materials and costs less.
  • It’s easier and more accurate to manufacture steel into a circle than any other shape.
  • A round manhole cover has a smaller surface than a square one and thus is made of less material and costs less to make (like Ledo’s Pizza slogan – “We’re square because we don’t cut corners!”).
  • Round manhole covers require less manpower and lifting to move. (Rolling, rolling, rolling!)
  • Cylinders are the strongest shape against the earth’s compression. Pipes are cylindrical, so it’s only natural that the manhole used to put the pipes into the ground is round, and that the cover for the manhole also assumes a circular shape.
  • The term “manhole” implies that the hole must be large enough for a man (or a woman) to pass through. Humans are roughly cylindrical in shape. So the hole and its cover should be too.

Remember, these type of interview questions propose a problem that has many possible solutions. There isn’t a “right” one, but rather ones that are “more right” than others. This is testing your creative and logical thinking abilities under pressure. Consider the problem from many perspectives, develop possible explanations, and then effectively communicate how you arrived at the most plausible solution.

Sound Off: Why are manhole covers round? Bonus points if you can address what we alluded to earlier: since not all manhole covers are round, why are the manhole covers that are round, round?

Try Another Deductive-Reasoning Problem: If a tree falls in a forest and no one is around to hear it, does it still make a sound? (Hint: Your answer depends on your definition of the word “sound.”)

Blogging Forward,

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Cartoon Credits: Implementing Scrum, David Farley
1Some responses published on Wikipedia. Others heard around CTC HQ!