How it works
Begin with a filled square. Divide it into a 3 × 3 grid and remove the middle square. Repeat inside each of the eight surviving squares. The generator draws filled cells so that the removed areas remain genuinely empty.
Where it began
Wacław Sierpiński introduced this construction in 1916. His paper on curves containing an image of any given curve appeared in Matematicheskii Sbornik. It is the square-based relative of the better-known triangular sieve.