This one ! You see these ‘manholes’ everyday right? on the surface of the roads??
Have you ever wondered why these took the shape of circle? Why not rectangle, square, triangle etc etc ? If you know the answer, please close the tab and surf for the other good answers :D
Think
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Its because the round shape cannot slip through the opening.
And, in other shapes there are inequalities. Diagonal of the square/rectangle shape would prove to be troublesome, it may slip into it and get struck etc.
Moreover, it need not be aligned every time when it needs to be closed. We can just close it so easily without any particular position. Just inserting it, that’s it! Work Done.
Image Source : Google Images
Content Source : My friend enlightened me about this concept