Continuing with my fascination for altered time and space perception (through mechanical means), I had not heard about Schlieren Photography until I saw this video on Slashdot Review over the weekend.
The balloon (what is it about balloons?) was one of my favorites, but the towel was cute. The shockwaves emanating from piston shots was instructive. In some cases you can actually see the bullet overtaking the shockwave from the initial firing of the weapon.