This is a game for two players. You will need a square grid to play on - a 9 x 9 grid is a good size.

The first player makes a mark on any corner.
The second player makes a different mark on another corner.

(It helps if each player uses a different colour)
The winner is the first player who can join his/her own marks to form a square.
Squares can be any size and can even be tilted over.

Not as easy as it sounds! Look carefully!