A water vortex in two soda bottles joined vertically.

Vortex is created in water draining from one bottle to another. Two plastic bottles are joined at the neck. Colored water almost fills the bottom bottle. Inverting the setup, and giving the liquid a rotary spin causes a vortex to form, draining the water quickly from the top to bottom bottle. (Without the rotary motion, the water slowly percolates down.) SURFACE TENSION AND CAPILLARY ACTION.