Example
A strategy-stealing argument for tic-tac-toe goes like this: suppose that the second player has a guaranteed winning strategy, which we will call S. We can convert S into a winning strategy for the first player. The first player should make his first move at random; thereafter he should pretend to be the second player, "stealing" the second player's strategy S, and follow strategy S, which by hypothesis will result in a victory for him. If strategy S calls for him to move in the square that he chose at random for his first move, he should choose at random again. This will not interfere with the execution of S, and this strategy is always at least as good as S since having an extra marked square on the board is never a disadvantage in tic-tac-toe.
Thus the existence of an infallible winning strategy S for the second player implies the existence of a similarly infallible winning strategy for the first player, which is a contradiction since the players cannot both have infallible winning strategies. Thus no winning strategy for the second player exists, and tic-tac-toe is either a forced win for the first player or a tie. (Further analysis shows it is a tie.)
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