Ice

In liquid water, hydrogen bonds are constantly stretching, bending, and breaking as the molecules rotate and jump around. The average lifetime of a hydrogen bond is about one picosecond (10^-12 seconds) in liquid water at 25°C. It is this network of flickering hydrogen bonds that gives liquid water its unique properties. This flickering also accounts for the fact that while water is more dense than ice, the "collapsed" structure is still "open" because of the highly directional character of the hydrogen bonds.