Tic Tac Toe

Tic Tac Toe needs no explanation and almost no instructions: two players alternate placing X and O on a 3×3 grid, and the first to line up three of their own marks wins. It takes about twenty seconds to play and roughly the same to master — which is exactly why it survives.

Three modes are available. 2 Players passes the device back and forth between two people. vs Easy plays a computer opponent that mostly moves at random and can be beaten comfortably. vs Hard plays a computer that searches the entire game tree before every move; against it the best possible result is a draw, and it will punish any error immediately.

Wins and draws are tallied across the session, so a two-player match becomes a running series rather than a single game.

1. Pick a mode. X always moves first; in the computer modes you are X.
2. Click or tap any empty square to place your mark. The scoreboard above the grid tracks X wins, draws and O wins for the current session.
3. Line up three of your marks in a row, column or diagonal to win. The winning line is highlighted in amber.
4. If all nine squares fill with no line, the game is a draw — the most common result between two players who know what they are doing.
5. Press Play Again to reset the board. The scoreboard carries over; switching mode resets it.

One control: click or tap an empty square. Occupied squares are disabled, so a mistimed tap does nothing rather than overwriting a move. There are no keyboard shortcuts and nothing is timed.

In computer modes the opponent replies after a short pause of about a third of a second. That delay is deliberate — an instant reply makes the board hard to read, and this way you actually see what changed.

• Take the centre if it is free. The middle square is part of four of the eight winning lines; every corner is part of three and every edge only two.
• Then take a corner, not an edge. Opening on an edge as the second player is the most common way to hand the game away.
• Look for the double threat. You do not win by making one line, you win by creating two threats at once so your opponent can only block one. Every real Tic Tac Toe win is a fork.
• Against Hard, aim for the draw. The computer searches every possible continuation, so there is no trap to spring. Play centre or a corner, block every threat, and you will draw every time.
• Against Easy, bait it. It plays essentially at random, so build a fork and it will usually fail to block the right square.