Tic-tac-toe, also known as noughts and crosses or Xs and Os, is a paper-and-pencil game for two players, X and O, who take turns marking the spaces in a 33 grid.
The first player to get three of her marks in a row (horizontally, vertically, or diagonally) wins the game. Tic-tac-toe is a simple game, but it can be surprisingly difficult to win. In fact, if both players play optimally, the game will always end in a draw.