The molecule structure of frozen water needs to bond more loosely in order to satisfy way the molecular structure is known to water. It is a phase transition ' Freezing water is an example of a phase transition -- a change in the physical properties of a substance when the temperature or pressure are changed. Phase transitions are often accompanied by either the absorption or release of thermal energy. Water molecules have electric dipole moments -- the oxygen atoms are more negatively charged than the hydrogen atoms, and the molecule is in a bent shape, with hydrogen atoms not quite on opposite sides of the oxygen. This means that water molecules strongly attract each other electrostatically (opposite charges attract each other). If there isn’t too much random motion of the molecules (that is, the water isn’t too hot), then the molecules prefer to line up in an orderly fashion, with the positively-charged part of one molecule next to the negatively-charged part of another molecule and so on, held together in a rigid crystal. If the molecules have more thermal energy, they shake around and break free of their neighbors. They still like to stick to one another, but because they are moving so much, they constantly change their neighbors and bounce off of each other. This is the liquid phase. ' Source: http://van.physics.uiuc.edu/qa/listing.php?id=1730