Let's have a thought experiment as an example of diffusion. If we drop an ink dot in perfectly still water, ink molecules will diffuse into the water spherically whose pattern is described by Fick's law of diffusion and forming a sphere of ink with a concentration gradient high in the origin and Gaussian decays further away, resembling the famous Maxwell-Boltzmann probability distribution along the spherical shells from the origin. For each ink molecule from the origin, the probability of it spreading into a nearby space with a distance r is mapped out by these many ink molecules and has been simplified into the equation with two parameters under ideal situations, the diffusion coefficient D and the elapsed