However if a "source" of liquid is inside the closed surface, such as a pipe through which liquid is introduced, the additional liquid will exert pressure on the surrounding liquid, causing an outward flow in all directions. This will cause a net outward flow through the surface "S". The flux outward through "S" equals the volume rate of flow of fluid into "S" from the pipe. Similarly if there is a "sink" or drain inside "S", such as a pipe which drains the liquid off, the external pressure of the liquid will cause a velocity throughout the liquid directed inward toward the location of the drain. The volume rate of flow of liquid inward through the surface "S" equals the rate of liquid removed by the sink.