All living things require water. The behavior of a water molecule is determined by its structure. Water consists of two hydrogen atoms covalently bonded to one oxygen atom. It is a polar molecule because it has oppositely charged sides. One side of the water molecule has a weak positive charge while the other side has a weak negative charge. The covalent bonds (shared electrons) between the hydrogen and oxygen atoms produce a molecule that has a tetrahedral, or triangular pyramid, shape. A covalent bond is a chemical bond that forms when atoms share electrons. The oxygen side attracts electrons to a greater degree than the hydrogen side, which draws electrons away from the hydrogen atoms. This leaves each side with a partial charge (positive on the hydrogen side, negative on the oxygen side). Like two bar magnets, whose oppositely charged ends are attracted to one another, two water molecules are attracted in a similar way.As a polar molecule, water does not interact well with any nonpolar molecule, whose sides carry equal or no electrical charges. Water molecules cannot easily form hydrogen bonds with nonpolar molecules. This means that nonpolar molecules do not dissolve well in polar solvents, such as water, acetic acid, and ethanol. Lipids—such as fats, steroids, and oils—are generally not water-soluble. This is why a drop of oil falling onto a wet surface fans out and forms a colorful pattern. Nonpolar substances need to be dissolved using other nonpolar solvents, such as diethyl ether and turpentine.