Why does water expands on freezing?

Water is an exception because most elements' molecules are more tightly packed when they are solidified than when they are liquidized.
As you may know, each water molecule is made up of one oxygen atom with two hydrogen atoms attached. The hydrogen atoms in water have a positive charge and cause the water molecules to be attracted to one another. As the water freezes, this attraction causes it to form a net of crystals with many gaps.
The crystal net structure takes up more space than liquid water and so water expands as it turns to ice.
(in detail):
Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink. The liquid contracts as it is cooled; because the molecules are moving slower they are less able to overcome the attractive intermolecular forces drawing them closer to each other. Then the freezing temperature is reached, and the substance solidifies, which causes it to contract some more because crystalline solids are usually tightly packed.
Water is one of the few exceptions to this behavior. When water is cooled to a temperature of about 4 degrees Celsius, it begins to constrict as one would expect. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%.
The structure of the water molecule is the source of this peculiar behavior. There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position). The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very "open"