You also have to watch where the computer puts its O. That could change what you do next. If the computer gets two Os in a row, you have to put your next X in the last empty square in that row, or the computer will win. You are forced to play in a particular square or lose the game.
If you always pay attention and look ahead, you'll never lose a game of Tic-Tac-Toe. You may not win, but at least you'll tie.
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