 Like many other substances, water cannot simply be described as a collection of individual molecules. Here are just a few examples of the complexities of its microstructure: water self-ionizes, which means that hydrogen and hydroxide ions co-exist with H2O molecules in liquid water, continually recombining to form H2O molecules. At the same time, the H2O molecules associate into larger polymeric species. Mentioning these complexities isn’t just pedantic because they are often what give rise to the most striking characteristics of substances. For example, the electrical conductivity of water is due to a mechanism in which a positive charge (hydrogen ion) attaches at one point of a polymeric cluster, inducing a co-ordinated transfer of charge across the cluster, releasing a hydrogen ion at some distant point. The effect is that charge is transferred from one point to another without a transfer of matter to carry it. The hydrogen bonding underlying the formation of clusters is also at the root of many other distinctive properties of water including its high melting and boiling points and its increase in density upon melting. As van Brakel has argued (1986, 2000), water is practically the poster child for such “non-molecular” substances.