Diffusion is an important concept in elementary science education, especially because it supports (or seems to support) the notion of matter consisting of very small everyday particles (as opposed to a continuous substance).
A colleague of mine is writing an introductory science text for 8th graders. She is struggling to find a good way to introduce the reader to diffusion using an everyday example. We do not like the canonical example of propagation of kitchen smells or perfume, because it is not obvious how to rule out spreading by air convection (i.e. advection of the smelly substance).
So, what's the best example to explain diffusion to an 8th grader?