If a column of liquid is held in a narrow tube closed at one end and lowered into a dye, how long would it take for the dye to diffuse through the liquid? A really long time, as this simulation shows.

Key in the diffusivity and the time since lowering of the tube into the coloured cells and observe how the colours in the tube change showing how much dye has diffused into the liquid. For typical values of diffusivity in liquids, it could take YEARS for the dye to diffuse to the top!! (In this simulation, the column of liquid is assumed to be 2 cm tall.)