The diagram shows the middle of a tic-tac-toe game. It is known the game eventually ended with a win. Assuming that both players are equally skilled and do not miss any chances to block a row from being completed by the opponent, can you say which player is going to win this game – O or X?
Clearly it is X’s turn, since two O’s and only one X are placed already. X must play in the upper left cell to block O’s winning threat (Figure 1). O then plays in the lower left cell (Figure 2), and X cannot block both of O’s threats. Thus, O wins.