Water is regarded as an unusual liquid. A prominent example of unusual behavior is that ice floats on liquid water, whereas most solids sink in their liquids. We have developed a simple statistical mechanical theory for the structure and physical properties of pure water, and of water as a solvent for nonpolar and ionic solutes. We find that hot water is a relatively normal liquid -- it expands with temperature for example -- but that the remarkable properties of cold water (room temperature or below) result from restrictions of water orientations due to hydrogen bonding. A very simple model can capture many of these properties.