India, Aug. 16 -- Have you ever walked past a manhole and wondered why its cover is almost always round? While there's no universal rule that manhole covers must be circular, round covers are far more common than square, rectangular or triangular ones. Read on to find out why. (Also read: Why do Indian sweets have silver layer on top? )

According to the American Water Works Association (AWWA), one of the biggest advantages of a circular manhole cover is that it cannot fall through its own opening. Unlike a square or rectangular cover, it has no corners that can be angled diagonally and slipped through the hole. This also makes installation easier. A round cover can be placed back over the opening without having to align specific corners ...