Tic-tac-toe is a boring game, once you get the hang of it. The only way to win is to play someone who hasn't mastered it, and mastering it is fairly easy. But you can change it up. Numberphile explains the variant Misère Tic-tac-toe, in which both players use X and the first to get three in a row is the loser. Then he explains how to win. Part two of the tic-tac-toe talk is at Boing Boing.