Reversibility of fluid motion in glycerin
Can be done because it is laminar flow, which means that the fluid is flowing is layers parallel to each other (in this case around the cylinder) without mixing. The diffusion on the dyes is slowed down because of the glycerin and then a flow is created by the spinning. Since there is no mixing, it is a (mostlty) reversible process, kind a like how you can integrate a known trajectory back in time to find out where something was thrown from. Turbulence in fluid is something that is not (easily/physically) reversible because of the mixing and the minimal diffusion.
This somehow is very cool and yet, makes me uncomfortable.
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