The game is played on a grid that's 3 squares by 3 squares.
You are X, your opponent is O. Players take turns putting their marks in
empty squares. The first player to get 3 of her marks in a row (up, down,
across, or diagonally) is the winner. When all 9 squares are full, the game is
over. If no player has 3 marks in a row, the game ends in a tie.
