Have you ever attempted to adjust the water temperature while you are in the shower and had trouble getting it just right? The water, let's call it the "fluid", is a mixture of two: hot water and cold water. Depending on the design of the shower, there could be two independent valves or a single mixing, or "3-way", valve to adjust. All together, the valve or valves (usually out of sight in the wall), the adjustment feature (knobs or levers), and the actuator (you) is the control valve. Standing there, wet, you are sensitive to the control valve application as the temperature swings from too hot to too cold.