  Imaginary assemblies of intercommunicating bottles-or boxes, in which marbles
circulate performing random walks, can help modeling (and hence understanding)
slow and long-lasting diffusion processes and enable easy evaluations of
typical times involved in their dynamics. Compared to the more traditional
approaches to explaining diffusion inspired by the Ehrenfest's urn problem,
these simple random walk models offer a more straightforward way to illustrate
irreversibility/recurrence issues.
