The generalization of fluid dynamics from pure to multicomponent fluids (fluid mixtures composed of different components or species) requires the introduction of new concepts, some of which are rather subtle and are less widely appreciated than they deserve to be. The purpose of this paper is to provide a simple didactic introduction to some of these concepts based on a detailed analysis of the equations governing the flow of ideal gas mixtures. The treatment is based entirely on a continuum description and makes no explicit use of the kinetic theory of gases. We include a straightforward and physically transparent derivation of the additional heat flux arising from the relative motion of the different species, and show why this flux involves species enthalpies rather than energies. Some of the concepts are reminiscent of those used in turbulence modeling, and these analogies are briefly discussed.