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The behavior of rules (c) and (d) from the previous page, starting with very simple initial conditions. Each panel shows 500 steps of evolution, and rapid randomization is evident. The black and gray cells behave much like physical particles: their total number is conserved, and with the particular rules used here, their interactions are reversible. Note that the presence of boundaries is crucial; for without them there would in a sense be no collisions between particles, and the behavior of both systems would be rather trivial.