Why are manhole covers circular?

Solution: For years in the early 1980s, job candidates coming for their job inter-views would yell upon arriving on the Microsoft campus, “Because they won’t fall into the hole!” And the puzzle still shows up because it’s a classic. Interviewers get good clues about the candidate even if he or she has studied the classic responses. Here are the classic responses, in order of their popularity:

  1. Circular covers are the only geometric shape that won’t fall into the manhole shaft they cover.
  2. Circular manhole covers don’t need to be aligned or oriented with the manhole shaft.
  3. Circular manhole covers make it easier to lift, carry, or even roll the heavy items.
  4. The manufacture of circular manhole covers is cheaper because it requires less metal than the manufacture of covers of any other shape.
  5. Because manholes are circular!

The puzzle really extends beyond just picking an answer. A candidate is expected to pick one or two responses and then defend them. A Microsoft recruiter found the following response perfectly reasonable for its grounding in moral values. It’s always good to remind the interviewer that these real-world problems are focused on preserving human safety:

A circular cover is the only geometric shape that won’t fall into the hole and possibly injure someone below or create a hazard for someone above. The slight lip on the manhole cover shaft prevents the cover from ever falling in, no matter how it’s held. A square manhole cover just wouldn’t work as well. That’s because the diagonal of a square is the square root of 2 times its side. Should a square manhole cover be held near-vertically and turned just a little, it would fall easily into the shaft. The same is true with triangular shapes. Circular shapes alone, because a circle has the same diameter in all directions, have the property we are looking for.

If you want to go with another response, fine. All these responses can work, so pick one and be prepared for the objections and conversation. This puzzle is not about your engineering skills. It’s about evaluating your ability to make a decision, communicate your reasoning, and defend it.

Avoid the temptation to be a smart aleck. Yes, there are other shapes besides circular that satisfy the condition that the cover be unable to fall into the shaft. But bringing the class of shapes referred to as Reuleaux polygons into the conversation will not advance your chances unless you’re applying for a topology position.




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