A really neat technique for visualizing the flow of fluids is called Schlieren flow visualization (pronounced: sh-lear-in). By using either special mirrors or lenses, the density gradients of a fluid can be seen. Examples would be the heat rising from a candle, or the aerodynamics effects of a bullet traveling through the air. This technique is used in wind tunnels and in many other areas of research.
This diagram from NASA shows a typical duel mirror Schlieren setup:
Here are a couple examples of what a Schlieren setup can produce:
And this one: