Ah the famous Reuleaux triangle, something which 99% of software developers around the world deal with every day!

Lots of man-hole covers are rectangular though. Turns out it isn't very important at all to have manhole covers that don't fall in. They are quite heavy and don't roll around on their own, so having them fall somewhere really is of minor concern.

Yeah, but they don't sound as nice when you hit them with the hammer (even when you make them of width/length golden ratio so that they look nice).

The manhole cover question, is more of a check to test whether the candidate belongs to the math-circle people where this is well known.

Even if they don't it's then an opportunity to test if the candidate can see the question behind the question.

With these sort of open-question in an interview context you have to roll with it, otherwise it's seen as an unwillingness to play (sphericon) ball.