Tic-tac-toe

Tic-tac-toe (or Noughts and crosses, Xs and Os) is a pencil-and-paper game for two players, X and O, who take turns marking the spaces in a 3x3 grid. The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal row wins the game.
Players soon discover that best play from both parties leads to a draw (often referred to as cat or cat's game).
The friendliness of Tic-tac-toe games makes them ideal as a pedagogical tool for teaching the concepts of good sportsmanship and the branch of artificial intelligence that deals with the searching of game trees.