Grease is a collective term for oils and fats. Chemically speaking, oils and fats are esters of higher fatty acids and glycerol. Greases are derivatives of hydrocarbons. Grease is a special kind of ester. The oil in nature is a mixture of various substances, the main component of which is the ester formed by the dehydration of one molecule of glycerol and three molecules of higher fatty acid, called triglyceride. The main physiological function of lipids is to store and supply heat energy, and the energy that can be provided in metabolism is about twice as high as that of carbohydrates and proteins. When one gram of fat is completely oxidized in the body, approximately 39.8 kilojoules of heat energy can be generated.