Water is simple, isn't it?

There is nothing simpler than water as a molecule. Its chemical formula, H2O, is almost the first thing in chemistry that one learns in school. However, its structure in the bulk is multifarious and changeable. There are 13 known crystalline structures of ice that appear under different temperatures and pressures. As a liquid, water forms dynamic 'flickering clusters' or networks of joined up molecules, with intermolecular bonds that flicker on and off at random. The basis for all this complexity lies in the ability of a water molecule to join up with its neighbours through a special kind of chemical bond, the hydrogen bond.