If simulation runs slowly, press minus key on title screen to reduce starting planets.
A N*N square starts out completely blocked (black). Locations are randomly unblocked (gray) until a water source from the top can traverse from the top to the bottom through adjacent unblocked locations, at which point the water has “percolated” through the square.
Click empty grids to add sand and click filled grids to remove sand. Note that removing sand creates the same behavior as adding sand, only upside-down.
This implementation is too inefficient for a larger grid.