The partially negative oxygen atom and the partially positive hydrogen atoms make the water molecule polar and capable of forming other bonds, specifically hydrogen bonds.
When water begins freezing, it first contracts.
The colder water becomes, the less kinetic energy
available to draw the molecules together. A hydrogen
bond is formed between the partially positive hydrogen
of one molecule and the partially negative oxygen of
another. These series of hydrogen bonds creates a crystal
structure. In actuality, the crystal structure is formed
by six molecules of water. For simplicity I use four.