Heat sinks are thermally conductive, usually metallic components or devices that absorb and dissipate heat generated by electronic components such as computers. Common heat sink materials include aluminum, copper, and steel. Larger heat sinks hold more heat than smaller heat sinks because of the relationship between surface area and the ability to dissipate heat radiantly. To help increase surface area, some heat sinks include integrated fans that use directed fins to achieve specific airflow geometries. These active heat sinks or fan sinks are typically used in CPUs as coolers and may be designed for specific processors. Passive heat sinks, such as thermal pads, do not contain integrated fans or other electronic components. Heat sinks include liquid coolers, which use a liquid medium such as water or liquid nitrogen to remove heat from components, and thermo-electric coolers, which use an electric element to pump heat from the element’s hot side to its cool side and transfer heat from the component to the environment. Vapor coolers or phase change coolers are similar to air conditioning or refrigeration systems, but evaporate a refrigerant in a small block mounted directly on the component.