Players take turns in marking squares on a grid. The first player unable to move loses.
The game is played on a grid; 6 x 6 is a good size. One player is 'O' and the other is 'X'.
Players take turns in writing their symbol in a cell. The restriction is that you can only play in a cell if all its neighbours are empty (shown shaded in the following diagrams).
The first player unable to move loses.
For example, in the following game the second player 'X' wins because 'O' has nowhere to play:
The game was invented and analysed by László Kozma, a Romanian mathematician. It is described on his web site here:
If both the height and width of the board are odd numbers the first player can easily win by starting in the middle square, and then playing the mirror image of the second player's move.
On even by even or even by odd sized boards there is no known winning strategy, and it makes for a challenging game.