Field
Flows and fields
Streamlines, particle paths and the field that carries them. What is conserved, what a picture of a flow can show, and what it cannot.
Streamlines are not the paths particles take
Three different curves get drawn through a flow and they are routinely treated as one. In steady flow they coincide, which is why the confusion survives; in unsteady flow they are as different as a photograph and a long exposure.
Mass has nowhere to go
Squeeze a stream of fluid and it speeds up, not because anything pushes it but because the same amount has to get through a smaller gap every second. Almost every result in the subject is that observation with more machinery attached.
What a flow is
A fluid is made of molecules and nobody models it that way. Treating it as a continuous field with a velocity at every point is an approximation, an extremely good one, and knowing why it works is knowing where it stops.