You probably know how to play Tic Tac Toe (or Noughts and Crosses or Xs and Os as it is sometimes called) from your childhood days.
If you want to learn more about this paper-and-pencil game, check out the Wikipedia entry: https://en.wikipedia.org/wiki/Tic-tac-toe.
You can play this game online at http://playtictactoe.org/ or at http://www.calculatorcat.com/games/tic_tac_toe.phtml.
The basic game of Tic Tac Toe is a competitive activity. Can you come up with a variation that is a cooperative game? Make sure the game will be engaging enough for two (or more) people to play.
Send your cooperative version to me (firstname.lastname@example.org) with the "Cooperative Tic Tac Toe" as the subject line. If our play-testing indicates that it is interesting, I will publish it (under your name) in a future issue of the Thiagi GameBlog.