Equipment:
Theory: As the water drains, gravitational potential energy is transformed into kinetic energy and the angular velocity increases. The liquid does not move into the center of the vortex because there is no force large enough to produce the required centripetal acceleration, so the vortex is stable.
Description: This is very easy to set up. Simply fill one of the soda bottles 2/3 of the way up with water, screw on the tornado tube that joins the bottles, and screw the empty second bottle on top. Place the bottles on a flat surface, invert, whirl the top bottle around a few times and you should have a nice vortex.
Setup time: 5 minutes