The weathering of rock said to occur when water, which has penetrated joints and cracks, freezes. In a bright and breezy paper, Murton (2007, Planet Earth 4) finds that the long-held explanation that freezing, expanding water shatters rocks is ‘probably not very significant in nature because it requires some pretty unusual conditions. The rock must essentially be water-saturated and frozen from all sides, to prevent the piston-like effect of freezing water driving the remaining liquid water into empty spaces or out of the rock through an unfrozen side or crack. So we need to look for another explanation.’