How to Play Tic-Tac-Toe

The rules

Mark squares in turn on a 3×3 grid; three in a row wins. The easiest game in the world to learn — with a proof hiding inside it that perfect play always draws.

Where it came from

Boards are scratched into temple roofs in ancient Egypt, and Rome played it as terni lapilli, "three pebbles." A few thousand years old, and playable anywhere you can scribble.

How to win

Start in a corner. Open in a corner and punish any mistake with a fork — two threats at once. With perfect play the game is always a draw, so a draw against the hard CPU is really a win.

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