A valve is simply a device that adjusts the flow of a fluid or gas. It works by opening, closing, or partially blocking the passageway where it is located. Most valves are used as pipe fittings; as such, they are a basic element in plumbing construction. Very easily recognizable are the plumbing valves in homes, which are used as taps for hot and cold water. Other familiar forms of valves are those found in cookers as gas control valves, and those in washing machines and dishwashers.