Inside the source and sink, source gives out fluid and sink takes in fluid, but as if the boundary is created when you put it in a uniform stream, as if a boundary is created around which the fluid goes past it, fluid cannot enter this place, is that clear? So, if I have a source and sink, this is very interesting, because it generates a body; now, you have got a body and this body can be represented by a source and a sink, and it satisfies all those conditions that we discussed.