Why are manhole covers round?


This question became famous when Microsoft began to use it as a job interview question and let’s face it, had you ever considered it before? Manhole covers are just there for you to walk over them. You notice them, you know they’re round, but asking yourself why didn’t seem like a big deal, did it? Well, Microsoft thought of it and its future employees had to come up with the answer. Imagine being in one of those interviews. Everything is going great and then the question pops out: “Why are manhole covers round?” and your whole existence begins to shatter. What? Never thought about it.

why are manhole covers round 1 303x227 Why are manhole covers round?

8 Reasons why manhole covers are round

Alright, so the question popped out and now you’ve got a few seconds to start talking before looking like an idiot to this important job interview. The reasons are:

1) It cannot fall through its circular opening. Not a bad idea. Think of a square manhole cover being inserted diagonally into the manhole. Yep, it could fall in and that’s bad news for nonobservant pedestrians or drivers. A round manhole cover cannot fall through its circular opening no matter how hard you try.

2) Easier to manufacture.

3) Round is the answer against the compression of the Earth. Round tubes are the most resistant because the pressure is evenly distributed on their surface.

4) Easier to put in position. Round manhole covers do not need extra rotating activities to place into position like, say, a triangle manhole. You just let it fall into the manhole and it will cover it perfectly.

5) Easier to transport. You can move them from place to place just by rolling them.  No need to lift them up and carry them as they are also quite heavy. They are made of cast iron, concrete or both. They aren’t expensive at all and they’re strong and heavy, weighing somewhere around 113 kilograms. Take that!

6) Because manholes are round. That’s a fact. It didn’t seem to bother anyone. It’s common ground. Maybe because the smallest circle a man can climb through is smaller than the smallest square. So, round is a much more economical shape. Or maybe it’s just because that’s how we built them and no one cared to do anything about it, right?

7) It’s good for GPS data. It seems that robots can calculate geographic position based on the round shapes of manholes and double-check the GPS data.

8) Better lock. A round manhole cover can be better locked into position and it is not easy to be opened if you don’t own a special tool for the job. Also, a round cover is hardly prone to being sucked up by traffic.

Why is this even a question?

Apparently is one of those questions that’s meant to determine a person’s ability to think fast and best on the spot. It is also one of those questions that seem so simple, but it isn’t. You’ve lived all your life with this kind of manholes and never asked yourself about them and now, in a few seconds, you have to come up with an answer without Wikipedia, Google or anything like that. Tough life, right?  Microsoft, Google and all the rest asking these types of questions are probably looking for that inquisitive mind always functioning, always wondering, always finding and solving problems. Otherwise, I don’t see how the matter of round manholes could really make a difference in a programmer’s life working at Google.

Anyways, you’ve got your answers now and if you are sure they are looking for some fun and imaginative questions, maybe try: “Because the aliens cannot see round shapes”.