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Why are manhole covers round?


Answer #1
This is a classic brainteaser, which was reportedly first asked by a Microsoft interviewer. Here's how to ""solve"" this brainteaser (remember to speak and reason out loud while solving this brainteaser): Why are manhole covers round? Could there be a structural reason? Why aren't manhole covers square? It would make it harder to fit with a cover. You'd have to rotate it exactly the right way.
The pipes below are also round, so fitting them might be easier, as might be making them. So many manhole covers are round because they don't need to be rotated. There are no corners to deal with. Also, a round manhole cover won't fall into a hole because it was rotated the wrong way, so it's safer. Looking at this, it seems corners are a problem. You can't cut yourself on a round manhole cover. And because it's round, it can be more easily transported. One person can roll it.

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Suppose there are three light switches outside a room. Inside is a single light bulb, controlled by one of the three switches. You need to determine which switch operates the bulb. You can turn the switches on and off as many times as you wish (they are all off to begin with), but may only enter the room once. There is no one there to help you. The door to the room is closed, and there are no windows, so you cannot see inside. How can you discover which switch operates the bulb?What can you offer us that someone else can not?