Puzzles centered around tic-tac-toe (a.k.a. noughts and crosses).
Tic-tac-toe, or 'Noughts and crosses', is a paper-and-pencil game for two players, who take turns marking the spaces in a 3×3 grid with the symbol 'O' or 'X', respectively.
The aim of the game is to place three of one's own marks in a horizontal, vertical, or diagonal row. If both players play optimally, a game will always result in a draw.