The thermodynamic behaviour of water seems to be related to static heterogeneities. These static heterogeneities are related to the local structure of water molecules and, when properly characterized, may offer an economical explanation of thermodynamic data. 'What matters' most in determining some of the unusual properties of liquid water may be the fact that the local geometry of the liquid molecules is not spherical or oblong, but rather tetrahedral. In respect to static heterogeneities, this local geometry is critical. The dynamic behaviour of water seems to be related to dynamic heterogeneities, which seem to explain the dynamics of supercooled liquid water well.