Instructions: Enter a rule number from 1 to 256 (#150 is pretty), and fill in some starting cells, then press go.
Explanation: The first row is determined by advance selection. Every subsequent row is filled in by following an algorithm that looks at the three nearest cells on the previous row. See Wolfram's numbering rules to understand how a rule number relates to the algorithm that chooses a colour for each cell. For example, rule 150 fills in a cell if the three cells above contain either 1 or 3 filled cells (but not 0 or 2).