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.
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The spin you need to start the sodas is pretty strong, a young child may not be able to do it by themselves. You could talk about tornadoes, water spouts, and such things while using this demo. As always when you present a demo make sure you ask the participants questions before you demonstrate and after. Think about the concepts involved beforehand; having a friend ask you questions about it can be very revealing as to if you understand it well or not. Whenever you ask a question count to five (or at least three) in your head waiting for an answer before you stay anything else.