- all the cells must be either black or white.
- numbers give the count of white cells in horizontal and vertical direction up to a border or a black cell.
- numbered cells are always white and included in above count.
- black cells can not be next to another black cell except diagonally.
- all white cells are connected (horizontal and/or vertical), never cut off by black cells.
- click on an empty cell to switch colors.
- click on a numbered cell for clues.