Aside: The material derivative 

Although we model fluids as if they are continuous materials, we can still identify blobs of fluid within that field. For example, we could drop a blob of ink into water and follow the ink as it moves around. If we knew the velocity field in the fluid, we could use the material derivative to calculate the velocity of that blob of fluid. This clip explains the material derivative by using the example of a punt drifting down the river next to the Mill pub, opposite Queen's College in Cambridge.