The purpose of lubricants, like motor oil, is to reduce the friction between surfaces so that parts last longer and less energy can be expended in the process of moving them. The key to a good lubricant is that the characteristic length scale for the formation of a thin film of the lubricant must be much smaller than the characteristic length scale of movement in the application. Basically, oils are good lubricants because they can form very thin films that persist even when the parts they serve to lubricate are constantly in motion. This ability to form thin films typically correlates with other properties that are easier to recognize. For example, good lubricants often have a high boiling point, low freezing point, high viscosity, and are stable toward chemical oxidation and changes in temperature.