Tap a square, or move with the arrows and confirm with space. You only ever hold three marks: place a fourth and your oldest lifts off the board (it dims and gets a dashed ring first), so the grid never fills and there are no draws. Line up three to beat the opponent — then a sharper one sits down, worth more points. Your shot clock shrinks with every move you spend, so stalling is fatal. Lose a round, or run the clock out, and it's over.