A heat sink uses its extended surfaces to increase the surface area in contact with the cooling fluid, the air for example. The term is not meant literally, as a heat sink does not have a "magical ability to absorb heat like a sponge and send it off to a parallel universe". Heat transfer theory helps explain practical aspects of how heat sinks work, and can also help to clear up common misconceptions and design mistakes. Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the design factors which influence the thermal resistance, i.e. thermal performance, of a heat sink. One engineering application of heat sinks is in the thermal management of electronics, often computer CPU or graphics processors. For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die temperature of the processor(s). Theoretical, experimental and numerical methods can be used to determine a heat sink's thermal performance.