Covalent bonds hold atoms together to form a molecule, but bonds between molecules are also important to life. For example, the characteristics of water molecules make them tend to stick together. The covalent bonds between oxygen and hydrogen atoms in a water molecule are polar-the atoms share electrons unequally. This results in the oxygen atom having a slight negative charge and the hydrogen atoms having slight positive charges. Watch what happens when two of these polar molecules get close to one another. A positively-charged hydrogen atom of one water molecule is attracted to the negatively-charged oxygen atom of another water molecule, drawing the molecules together. This weak, temporary attraction is called a hydrogen bond. Many biological processes depend on such weak, transitory molecular interactions, so hydrogen bonds are crucial to living things. Hydrogen bonding keeps protein molecules folded in their characteristic three-dimensional shapes. The two strands of the DNA double helix are held together by hydrogen bonds.