The type of bonding that holds two or more water molecules together is called hydrogen bonding. Water molecules are polar, meaning they have slightly negative and positive regions within the molecule.
The oxygen region of a water molecule has a slightly negative charge, whereas the hydrogen portions of the molecule take on slightly positive charges. When the partial negative oxygen portion on one water molecule meets up with the partial positive hydrogen side of another water molecule, a weak bond forms. One water molecule can hydrogen bond to four other water molecules. A hydrogen bond is relatively weak, having only about 5 percent the strength of a covalent bond; however, numerous hydrogen bonds are actually quite strong. The hydrogen bonding of water gives it unique qualities.