... except when they aren't. This reminds me of the famous "why manhole covers are round" question. They aren't, as a quick look at the Wikipedia page can conf

... except when they aren't. This reminds me of the famous "why manhole covers are round" question. They aren't, as a quick look at the Wikipedia page can confirm: https://en.wikipedia.org/wiki/Manhole https://en.wikipedia.org/wiki/Manhole