One of the four Aristotelian elements, water has played a central role in scientific thought for millennia. To the physical scientist it is a continuing source of fascination because of its many unusual and counterintuitive properties. For example, liquid water, if sufficiently cold, expands and becomesmorecompressible when cooled, andlessviscous when compressed. Water can also exist in at least two distinct glass forms—a phenomenon known as polyamorphism.