Used motor oil which has impurities and dirt is fed right into a centrifuge. The centrifuge carries a rotor that spins an incredibly high RPM (revolutions each minute) and it pushes the impurities to the wall in the container that houses the rotor. This works because the mass from the impurities is more than the mass with the oil particles. Appropriate extraction methods are used to ensure that impurities along with the clean motor oil are properly separated, and the oil is able to be re-used. These filtering systems can also be suitable for use with small-scale biofuel setups to get rid of any impurities. Sunglasses allow better vision in bright daylight, and may protect one's eyes against damage from high levels of ultraviolet light. Typical sunglasses are darkened for protection against bright light or glare; some specialized glasses are clear in dark or indoor conditions, but turn into sunglasses when in bright light. Most sunglasses do not have corrective power in the lenses; however, special prescription sunglasses can be ordered. Specialized glasses may be used for viewing specific visual information (such as stereoscopy) or 3D glasses for viewing three-dimensional movies. Sometimes glasses with no corrective power in the lenses are worn simply for aesthetic or fashion purposes. Even with glasses used for vision correction, a wide range of designs are available for fashion purposes, using plastic, wire, and other materials. So, as a contraceptive method, its success is limited! However, doctors do not recommend doing so because: