viscous flow

In a pure liquid, molecules touch one another but change neighbors frequently.  External forces cause the liquid to change shape by viscous flow.  Thermal agitation causes  molecules to undergo self-diffusion.  The two phenomena--viscous flow and self-diffusion--often result from a single rate-limiting process:  molecules change neighbors.  This simple picture is amply confirmed by the Stokes-Einstein relation, which links the viscosity and self-diffusivity for many liquids over wide ranges of temperature.

I just remembered another video that I had seen some time ago.  Many of you have probably seen it but here it is for those who have not. (The original page where I found it is http://www.maniacworld.com/Laminar-Reverse-Flow.html.)

Prof. Pat McMurtry's explanation is: