Chemists have long recognized water as a substance having unusual and unique properties that one would not at first sight expect from a small molecule having the formula H2O. It is generally agreed that the special properties of water stem from the tendency of its molecules to associate, forming short-lived and ever-changing polymeric units that are sometimes described as "clusters". These clusters are more conceptual than physical in nature; they have no directly observable properties, and their transient existence (on the order of picoseconds) does not support an earlier view that water is a mixture of polymers (H2O)n in which n can have a variety of values. Instead, the currently favored model of water is one of a loosely connected network that might best be described as one huge "cluster" whose internal connections are continually undergoing rearrangement.
Water has long been known to exhibit many physical properties that distinguish it from other small molecules of comparable mass. Chemists refer to these as the "anomalous" properties of water, but they are by no means mysterious; all are entirely predictable consequences of the way the size and nuclear charge of the oxygen atom conspire to distort the electronic charge clouds of the atoms of other elements when these are chemically bonded to the oxygen.