Line up five or more balls of one colour to clear them. Every move that doesn't clear a line brings three more.
Click a ball, then click an empty cell to move it there. Cells it can't reach are dimmed (or, in straight-line mode, the ones it can reach are dotted). Rows, columns and diagonals all count. Longer lines score more: 5 balls = 10, 6 = 24, 7 = 42. Keyboard: arrow keys to move around, Enter to select and move, Esc to deselect, U to undo. Your game is saved if you leave the page.