This Demonstration shows the formation of percolation clusters in elementary cellular automata with different colors. Also shown is a log-log plot of the size distribution. The first row in the graphic shows the initial spatial conditions, which can be random with a given occupation probability ("random"), a single occupied cell ("single 1") or a single empty cell ("single 0"). Try rules 30, 45, 60, 86, 99, 105, 129, 150, 151, 153, 169, 182, 183, 184, 195 and 225 under different occupation probabilities to see a strong linear relationship, whose slope, known as the power law exponent, has important implications in a wide variety of applications. Most of the slopes are less than 1, implying diverging means and variances of cluster sizes, which strongly suggests critical behavior.