The first row contains one coloured cell. Rule 30 defines the production of each subsequent row. Representing 'coloured' by 1 and 'blank' by 0, a cell in the next row is coloured if it lies directly below a sequence 100, 011, 010 or 001. In other cases the cell is left blank or made blank. Although this rule is completely deterministic, the central area of the grid produced has an appearance of randomness, and in fact the central column has been used to produce sequences of pseudo-random numbers. There is also an intriguing similarity to patterns on the cone shell.