Win three games of Tic Tac Toe in a row.
You may only play in the big field that corresponds to the last small field your opponent played. When your are sent to a field that is already decided, you can choose freely.
This is a pen and paper game – great for down time, slow time, think time.
Better explaination here at Math With Bad Drawings.
I now play this with my kids and friends when I am waiting for dinner or for anything.