Two bottles have been joined and one of them is full of water. If we turn them upside-down and then twist the top one with a movement similar to mixing pastry, we create our own twister.

The vortices that are formed as a result revolve either left or right depending on the way we twist. In this way we can bring about our own twister in a bottle. In general, vortices form when the speed of flow changes dramatically and the phenomenon of viscosity is of great importance. In the vortex created in the bottle the speed of the water near the sides is approximately zero, while the closer to the centre, the greater the speed of the water.

Convection currents are good example of vortices. They form in atmosphere on nice spring and summer days. The air in such currents travels in spirals and has the shape of a vortex. But as it get higher, this 'column' of rising air widens (as a result of adiabatic expansion), and cold air that surrounds this ascending current is the element that has something to do with viscosity. If the air gathers enough momentum a twister is formed.

Vortices are difficult to describe. It is because they are often formed as a result of turbulent flow. For low turbulent flows, the surface of vortex can be described with satisfactory approximation with the means of rotational hyperbola. In case of high turbulence flows, vortices multiply themselves. In a big vortex a smaller one is formed, in which even smaller one is formed and so on, as in nesting dolls (matrioshka), in a word - chaos. Apparently, Werner Heisenbeg said on his death bed, he would ask God two questions when he met Him: "Why relativity? Why turbulence?". After a while he added, "I really believe He will have an answer only to the first one." (J. Gleick, "Chaos. Narodziny nowej nauki." Wyd. Zysk i S-ka, Poznań 1996).

A vortex in a bottle allows for overcoming viscosity forces: were it not there, the air would squeeze through a small aperture between the bottles in small bubbles. Centrifugal force in swirling water pushes it to the edge of the aperture, allowing for a smooth passage of a stream of air through the centre of the aperture at the top.