Fatty acids are molecules made up mostly of carbon and hydrogen. The more hydrogen they contain, the more saturated they are. And the stiffer. When one pair of hydrogen atoms is missing from a fatty acid — the case with OLEIC ACID — it's said to be monounsaturated (mono simply means that one pair of hydrogen atoms is missing). When food processors turn vegetable oils into thick and fluffy shortenings, they do so by adding hydrogen. See also FAT.