The solid form of most substances is denser than the liquid phase so a block of most solids will sink in the liquid but a block of ice floats in liquid water because ice is less dense. Upon freezing the density of water decreases by about 9%. This is due to cooling of intermolecular vibrations allowing the molecules to form steady hydrogen bonds and locking into positions of hexagonal packing upon freezing to ice. These bonds are shorter in the crystal than in the liquid.