A computer fan is any fan used for active cooling inside or attached to a computer case. Fans are used to draw cooler air into the case from the outside, exhaust hot air from the inside, and move air through a heat sink to cool specific components. Axial-flow fans and sometimes centrifugal (blower/squirrel-cage) fans are used in computers. Computer fans are usually available in standard sizes such as 120 mm (most common), 140 mm, 240 mm, or even 360 mm. Computer fans are powered and controlled using 3 or 4 pin fan connectors.