In the solid state (ice), water molecules are held in a crystalline structure by hydrogen bonds. This structure is an open lattice containing hexagonal rings, with significant empty space or "interstitial holes" between the molecules. In the liquid state (water), although hydrogen bonds are still present and constantly breaking and reforming, the structure is less ordered and more compact than ice. As a result, a given mass of ice occupies a larger volume than the same mass of liquid water at the same temperature, leading to ice having a lower density and thus floating on water.