Tic-tac-toe

The player (X) plays against the Android App (O). The two players place their mark alternately on a 3 x 3 squares wide playing field. Who sets the first three of his characters in a row, column or main diagonal is the winner.
Skill level of your opponent can be adjusted.
Idea / theory: http://en.wikipedia.org/wiki/Tic-tac-toe