a curve of constant width won't fall in any orientation

So you could throw a bunch of Reuleaux tetrahedra on the floor,and slide a board across them as smoothly as if they were marbles.Now back to manhole covers.A square manhole cover's short edge could line up with the wider part of the hole and fall right in.But a curve of constant width won't fall in any orientation.Usually they're circular, but keep your eyes open,and you just might come across a Reuleaux triangle manhole.