How to Play Tic Tac Toe
Tic Tac Toe — noughts and crosses, or XO — is a three-by-three grid game where two players take turns placing their mark. The first to line up three of their own marks horizontally, vertically or diagonally wins. If the board fills with no line, the game is a draw.
Nearly everyone learns this game as a child and nearly everyone stops thinking about it shortly afterwards. It is worth a second look, because Tic Tac Toe is one of the few games that has been completely, mathematically solved — and understanding how it is solved is genuinely useful.
Controls
Tap or click any empty cell to place your X. The computer responds immediately with its O. There is nothing else to manage.
Three difficulty levels
- Easy — the AI plays random legal moves. It will miss obvious wins and fail to block you. Good for young children or a quick warm-up.
- Medium — the AI usually plays optimally but makes occasional mistakes. Beatable if you are paying attention.
- Hard — the AI runs a full minimax search and plays perfectly every single time. It cannot be beaten. A draw is the best result available to you, and earning that draw is the real challenge.
Why hard mode is unbeatable
Tic Tac Toe has a small enough number of possible positions that a computer can simply examine all of them. Minimax works by looking at every legal move, then every possible reply, then every reply to that, all the way to the end of the game. It assumes you will always play your best move, and it picks the move that gives it the best guaranteed outcome against that.
Because the entire game tree fits comfortably in memory, there is no approximation involved. The hard AI is not "very good" — it is provably optimal. With perfect play from both sides, Tic Tac Toe is a draw, which is why a draw against hard mode is the actual win condition.
The opening
If you move first, take the centre. It is part of four of the eight possible winning lines — the two diagonals, the middle row and the middle column — which is more than any corner (three) or edge (two). Centre is the strongest opening square by a clear margin.
If the AI takes the centre first, reply with a corner. Corners belong to three winning lines each and keep both diagonals live for you. Replying on an edge is the weakest response and loses against strong play.
Forks: the only way to win
Against an opponent who blocks reliably, a single line of attack never works — they simply block it. The way to actually win is a fork: a move that creates two separate two-in-a-rows at the same time. Your opponent can only block one of them, and you take the other on your next turn.
This is the entire strategic content of the game. Every win between competent players comes from someone creating a fork, and every draw comes from both players successfully preventing them.
Block before you attack
The habit that costs casual players most of their losses is getting so absorbed in building their own line that they miss the opponent's. Before every move, check the board for two of the opponent's marks in a line with an empty third square. If one exists, block it. Your own plan can wait one turn; a completed line cannot be undone.
A good first strategy game
Tic Tac Toe is often a child's introduction to thinking ahead, and it teaches the idea cleanly: your move is not just about what it does now, it is about what your opponent can do next. It is also a nice demonstration of why some games stay interesting and others do not. Once you understand forks and the centre-square rule, the game is finished — which is exactly why chess, with its unmanageably large tree, has survived a thousand years longer.
Plays on any device
Tic Tac Toe runs in your browser using HTML5 and works on phones, tablets, laptops and desktops. No download, no signup.
