Fluid dynamics is the study of how liquids behave while they are in motion. The study of this can become very complicated for many reasons. Fluids can be have steadily or with turbulence. In steady flow, the fluid passing a given point maintains a steady velocity. For turbulent flow, the speed and or the direction of the flow varies. In steady flow, the motion can be represented with streamlines showing the direction the water flows in different areas. The density of the streamlines are directly proportional with its velocity. Fluids can be compressible or in compressible. This is the major difference between liquids and gases, because liquids generally are in compressible, meaning that they don't change volume much in response to a pressure change; gases are compressible, and will change volume in response to a change in pressure. Also, Fluid flow can be rotational or irrotational. Irrotational means it travels in straight lines; rotational means it swirls. The dynamics of fluid and defining it movement can be very complex for many reasons.