Friction is created by relative motion between two surfaces. Lubrication uses a lubricant material between surfaces to reduce friction and resistance to motion. There are two main lubrication methods: hydrodynamic lubrication which uses a fluid film to completely separate surfaces, and boundary lubrication which occurs when surfaces are in contact and the lubricant forms an adhesive film. Lubricants can be liquids, solids, semi-solids or gases and are used to reduce friction and wear, transfer heat, clean parts, reduce noise, and create seals between surfaces in machines.