Cited from http://www2.physics.utoronto.ca/~nonlin/mud.html: Mud cracks when it shrinks and become brittle. If you make a uniform layer of mud and dry it uniformly, you get a pattern of irregular polygons with edges that usually meet at 90 degrees. If the layer is very thin, you get polygons with meandering "diffusive" cracks at the edges. In either case, contrary to popular belief, most of the polygons are not hexagons.