Mathematicians play a sophisticated version of Tic-Tac-Toe

Ben Orlin in Math With Bad Drawings:

In each square of their tic-tac-toe board, they’d drawn a smaller board:

As I watched, the basic rules emerged quickly.

Each turn, you mark one of the small squares.

When you get three in a row on a small board, you’ve won that board.

To win the game, you need to win three small boards in a row.

More here.