In this applet, a "disturbance" wanders randomly on a grid of colored squares. Initially all the squares are black. Each time the disturbance visits a square, that square becomes a slightly brighter shade of green, up to a certain maximum brightness. Eventually, one supposes, the entire square would become a uniform bright green.
To perform its "random walk," the disturbance picks a direction -- up, down, left, or right -- at random and moves one square in that direction. This is repeated indefinitely. (Note that the top edge is considered to be connected to the bottom edge, and the left edge is considered to be connected to the right edge. If the square moves off one edge of the applet, it reappears on the opposite edge.)