A Zerk, also known as a grease fitting, is a metal fitting used in mechanical systems to feed lubricants into bearings and other wear points. It works by acting as a one-way check valve that allows grease to pass through one way, but not the other. The Zerk fitting consists of a small nipple that threads into a hole and has a ball bearing in the tip which keeps the grease inside. To use it, attach a grease gun to the fitting and inject grease until it starts coming out of the bearing or other wear point.