The simple game where two players take turns drawing X or O in a grid of nine squares, and the winner is whoever gets three of their own mark in a straight line — across, down, or corner to corner. Children play it on paper, on a steamed-up window, anywhere. This is the American name; British speakers call the same game 'noughts and crosses' ('nought' being their word for zero). It is easy enough that two careful players always draw, which is why grown-ups often use it as a picture of something pointless: 'that meeting was tic-tac-toe.' You will also meet it in computer classes, because it is a favourite first example for teaching a program to play a game.