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This is the simplest cellular automata where we plant a seed at the centre of the cellular lattice, decide the type of neighbourhood in terms of the shape and number of cells which define the hinterland of influence on any cells (Moore, von Neumann and so on), and then determine how many cells in each neighbourhood activate the central cell in question: there are three choices, > 0, =1 and <=2.
We can also set the probability of this rule being activated from 0% to 100% and we can set an age limit in terms of time when the cell of that age is set back to undeveloped. There are various others ways in which you can experiment with this but we leave that to you. Note that this program is set to go and to get the regular diffusion you should not change the probability value from 100%.