Manhole Cufflinks {Luxist}

Sep 1st 2006 5:38PM Round isn't actually the only shape that won't fall in. You can also make them in a rounded triangle shape, where the side opposite a vertex is an arc of a circle with radius equal to the length of the side of the triangle.

Or, for those not math-inclined, like a triangle that's been inflated.

Despite the shape, covers do regularly fall into drains. Sometimes the collar that the cover sits on breaks. Sometimes the cover does. Though they are very heavy and very strong, they're cast steel, so they'll break instead of bending if too much weight is put on them.