Door closers work by using a spring tension modulated by hydraulic fluid. This allows the door to close slowly, but firmly, so it can latch shut, without springing closed on potential guests. When someone opens the door, hydraulic fluid moves from one reservoir to another, the spring then pushes the door closed, moving the hydraulic fluid back to the previous reservoir through a series of valves. The valves help control the speed of how fast the door shuts.