ELI5 Circular Windows - why don't planes and ships have square windows?

Why did the De Havilland Comet aircraft lead to a critical design decision? Why do planes and ships have circular windows instead of square ones? Why do airplanes have more windows than seat rows?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: lilmamameows, deep_sea2, crazykentucky, majasshole, peach-fuzz1, hiteshhedwig, concise_pirate & mjmurcott

Wikipedia reference: https://en.wikipedia.org/wiki/De_Havilland_Comet

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