each water molecule is two hydrogen atoms bonded to one oxygen atom. because of how the atoms share electrons, a water molecule is slightly positively charged at the hydrogen atoms, and slightly negatively charged at the oxygen atom. The molecule's charged ends attract the oppositely charged ends of other water molecules. In liquid water, as molecules slip-slide past each other, bonds form, break, and reform. But by the time water has cooled to 4 C., the molecules energy has dropped enough that they are very near one another. So each H2O molecule forms more stable hydrogen bonds, with up to four other molecules.