To illustrate the rocket principle in a dramatic way.
Two rocket bottles with liquid nitrogen.
Pour about 100-200 ml of liquid nitrogen into the bottle and install the stopper. Exhausting nitrogen gas and liquid result in motion of the bottle. A second bottle has an untethered stopper, for comparison.
David Kagan, Louis Buchholtz, and Lynda Klein, Soda-Bottle Water Rockets, TPT 33, 150-157 (1995).