Shade some cells black to create a snake - a chain of blackened squares which doesn't touch itself even at a point. Begin and end of the snake are already given. Numbers outside show the length of maximum blackened fragment 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.