A stopcock is a valve used to restrict or isolate the flow of a liquid or gas through a pipe. In Great Britain a stopcock, is used to prevent flow of water into a domestic water system. Laboratory glassware items sometimes have plug valves with conically-tapered plugs called stopcocks. When fused with the glassware, the valve bodies are made of glass. Otherwise, they can be made of an inert plastic such as Teflon. The plugs can be made of a similar plastic or glass. When the plug is made of glass, the handle and plug are fused together in one piece out of glass. When glass is used for both the stopcock body and the plug, the contacting surfaces between them are special ground glass surfaces often with stopcock grease in between. Special glass stopcocks are made for vacuum applications, such as in use with vacuum manifolds. They often have hollow plugs and glass tubing for the passageways across the side of the hollow plug. The valve body at the end opposite the handle is closed off, but the hollow plug is open at that end. This way, vacuum in the stopcock keeps the conical plug drawn into the conical seat in the body. Stopcock grease is always used in high vacuum applications to make the stopcock air-tight.