Tic-tac-toe, also known as noughts and crosses, is a two-player game typically played on a 3x3 grid. One player uses X's and the other O's, taking turns to mark an empty cell with their symbol. The objective is to create a line of three of their own symbols horizontally, vertically, or diagonally. If the grid fills up without a winner, the game ends in a draw. Tic-tac-toe is simple to learn, yet it can involve strategy and foresight to anticipate and block your opponent's moves while seeking opportunities to win. It's often used as a teaching tool for basic game theory and problem-solving skills.