You probably already know about how to play Tic-Tac-Toe. It's a really simple game, right? That's what most people think. But if you really wrap your brain around it, you will discover that this is not quite as simple as you think! The game is played on a grid that's 3 squares by 3 squares. You are X, your friend is O. Players take turns putting their marks in empty squares. The first player to get 3 of her marks in a row is the winner. When all 9 squares are full, the game .is over. f no player has 3 marks in a row ,the game ends in a tie.