It was a very simple way of demonstrating subsonic and supersonic flow by way of a 2D analogy of wave effects in a very thin sheet of water running down a more-or-less inclined sheet of glass.
Another useful water analogy is the hydraulic jump.
Use the tap/faucet in your kitchen sink to impact a jet of water onto the smooth, horizontal sink surface. The water jet radiates as a thin film of water until it enters the hydraulic jump region. Here, there is a flow discontinuity with the ring of the jump being analogous to a normal shock wave in a compressible gas. Plenty of pictures on the net of jumps.
Main generic things to keep in mind with intake flow -
(a) a normal (perpendicular to the flow direction) shock wave is bad news for flow parameters with lots of losses and so forth
(b) you have a normal shock as you transition through the transonic region whether you like it or not
(c) the trick is to use a series of oblique shocks ahead of the normal shock to step the flow changes progressively so that the abrupt changes across the final normal shock are reasonably minimised. This is the artform inherent in intake design and geometry.
(d) while doing the bits in (c), the designer can pick up lots of useful pressure distributions on the tinwork which is what the intake thrust discussions are all about.