Because they are easier to manufacture than other shapes which will not fall through the hole either: http://en.wikipedia.org/wiki/Reuleaux_triangle http://en.w

Because they are easier to manufacture than other shapes which will not fall through the hole either: http://en.wikipedia.org/wiki/Reuleaux_triangle http://en.wikipedia.org/wiki/Reuleaux_triangle Want to attract nerd tourists ? Build your city with Reuleaux manhole covers in the streets and Penrose tilings on the walkways.