Nature Mater. doi:10.1038/nmat2403 (2009)

As a snowflake's sixfold symmetry shows, ice crystals contain water molecules locked into hexagonal arrays.

But Angelos Michaelides at the London Centre for Nanotechnology and his colleagues have identified circumstances in which ice breaks this rule. Water molecules forming nanometre-wide chains on a particular copper surface at 100–140 kelvin are arranged in pentagons. The motif arises from a balance of hydrogen-bonding interactions between water molecules and the strain induced by fitting to the lattice of the copper surface beneath.