This is quite an interesting look at the basics of sound. Using a small explosion, it demonstrates that the speed of sound at sea level is around 760mph, but that varies as the altitude increases, the temperature falls and therefore can drop to c. 660 mph before entering the constancy of the stratosphere. What's that got to do with high speed aviation, I cry? Well, this well written and delivered documentary proceeds to explain the difference between sub sonic pressure waves and those travelling as fast or faster than sound allowing us to consider how it and speed are distinctly inter-related in a way not immediately obvious to the layman (me!). We are also presented with a basic guide to aerodynamics illustrating how the physical structure and weight of the aircraft can affect it's speed - delta wings, or crescent wings for example. I'm no scientist, but found this quite easy to follow for 20 minutes, with plenty of actuality of aircraft in flight as these problems are gradually being overcome, to keep it absorbing.