Particularly, within a specific volume; that means, the amount of fluid that will enter to a particular region, it will be a same amount of fluid has to will go out from the region.
Now so, let us derive this mathematically what does it says let us consider a to do that let us consider heavily fluid surface, and let me consider small this is the surface in the fluid a closed surface. Now, let us say the volume of the in this region the amount of fluid volume of the fluid is tau, and let us consider any elementary surface on this d s.