Oil is any substance that is insoluble in water, soluble in ether, and greasy to the touch. There are three main groups: mineral oils (see petroleum); fixed vegetable and animal oils (see fats; and volatile vegetable oils (see essential oils). Oils are classified as fixed or volatile according to the ease with which they vaporize when heated. Mineral oils include gasoline and many other fuel oils, heating oils, and lubricants (see lubrication). Fixed vegetable oils are usually divided into three subgroups depending on the physical change that occurs when they absorb oxygen: oils such as linseed and tung, which form a hard film, are known as drying oils; semidrying oils, such as cottonseed or soybean oil, thicken considerably but do not harden; nondrying oils, such as castor and olive oil, thicken only slightly. Fixed animal oils include the marine oils, such as cod-liver and whale oil. Fixed animal and vegetable fats, such as butterfat and palm oil are also classified as oils. Examples of volatile vegetable oils, which usually have a very distinct odor and flavor, include such oils as bitter almond, peppermint, and turpentine. When dissolved in alcohol, they are called essences.