Any organic compound of plant or animal origin that is not volatile, does not dissolve in water, and is oily or greasy. Chemically, fats are identical to animal and vegetable oils, consisting mainly of triglycerides (esters of glycerol with fatty acids). Fats that are liquid at room temperature are called oils. Differences in melting temperature and physical state depend on the saturation of the fatty acids and the length of their carbon chains. The glycerides may have only a few different component fatty acids or as many as 100 (in butterfat). Almost all natural fats and oils incorporate only fatty acids that are constructed from two-carbon units and thus contain only even numbers of carbon atoms. Natural fats such as corn oil have small amounts of compounds besides triglycerides, including phospholipids, plant steroids, tocopherols (vitamin E), vitamin A, waxes, carotenoids, and many others, including decomposition products of these constituents. Sources of fats in foods include ripe seeds and some fruits (e.g., corn, peanuts, olives, avocados) and animal products (e.g., meat, eggs, milk). Fats contain more than twice as much energy (calories) per unit of weight as proteins and carbohydrates. Digestion of fats in foods, often partial, is carried out by enzymes called lipases. The breakdown products are absorbed from the intestine into the blood, which carries microscopic fat droplets reconstituted from digested fats (or synthesized in cells) to sites of storage or use. Fats are readily broken downprimarily into glycerol and fatty acidsby hydrolysis, a first step for many of their numerous industrial uses. See also lipid.