I’m sure I’ve talked about stream lines and streak lines and all the other kinds of lines that you learn about in hydrodynamics at some point. The other day I saw a very nice (and slightly disgusting) example of trajectories.
The ducks paddling in muddy waters left distinct trajectories of where they had been. Additionally there was some flow, so the ducks as well as their trajectories were advected with the flow. Very interesting to watch!