In hydrodynamics, the behavior of flowing liquids is illustrated by flow lines. They run in the same direction as the flow itself. If the outflowing liquid hits an edge, the flow is compressed into a smaller cross-section. It only does not break off if the flow rate of liquid particles remains constant, regardless of where an imaginary cross section (perpendicular to the flow) is located. So the same amount of mass must flow in through one cross-sectional area as flows out of another. One can now conclude from this, but also observe in reality, that the flow accelerates at bottlenecks and the streamlines are bundled. This situation describes the continuity equation for non-turbulent flows.