Cork grease is a lubricant used to lubricate cork, for example in musical wind instruments. Meibomian glands in the eyelid secrete sebum to lubricate and protect the eye. Three accessory glands provide fluids that lubricate the duct system and nourish the sperm cells. These glands secrete oily fluid that lubricate and protect the nipple during breastfeeding. The salivary glands lubricate the mouth with moisture or spit. Salivary secretions lubricate the food and they also contain bioactive compounds. The oiling system addresses the need to properly lubricate an engine when it's running. Vaseline was used to lubricate the rotor disc contacts. The bearings decreased rotational friction by using mud to lubricate stone rings. Replace any worn parts, rekey, lubricate, and reassemble. Structurally, fuel was used to lubricate and cool the turbine bearings. Petroleum jelly can be used to lubricate zippers and slide rules. Other systems are also used to lubricate the cylinder and piston. Inks often contain fatty acids which help to lubricate the ball tip while writing. GAGs (formerly called mucopolysaccharides) are also found in the fluids that lubricate joints.