"Simple diffusion is like a random walk, Banavar explains. In such a walk, the direction of each successive step is governed by chance. If a crowd of random walkers starts from the same point, the pattern of dispersal of the crowd is predictable. At first, the walkers cluster relatively near the starting point, with only a few farther away. As time goes on, this bell-curve distribution becomes flatter and flatter until the crowd of walkers is evenly dispersed from the starting point, like the dye in the water bucket."