In this thread, I will explore some elementary concepts in fluid mechanics, starting with the distinction between a particle description and a field description (Lagrangian vs. Eulerian formalism). I will continue by distinguishing between the concepts of particle trajectories, streamlines, material curves and streak lines. Furthermore, I'll look into the basic derivations we may make with these concepts in inviscid flow, namely Bernoulli's equation, Crocco's theorem, Kelvin's theorem, Helmholtz' theorem for vortex lines,and the helicity theorem. Lastly, I'll look into the assumptions behind irrotational, i.e, potential flow, the generalized potential flow known as the Clebsch flow, and the elegant principle known as Luke's principle of variation. Quite a bit more to follow in a while..