Paint some cells black to form a coral - single connected area. No 2x2 squares can be completely blackened. The coral doesn't have any holes - all white cells should be connected to the edge of the grid. Numbers outside show the lengths of the first fragment of the coral in the corresponding row or column.
Grid control: Click the mouse button at the selected cell to change the color of the cell. You can use mouse dragging to copy content from one cell to another. More details about grid control are at the help page.