Square it!

https://nrich.maths.org/squareit

This game can be played against the computer, or against a friend.

Players take it in turns to click on a dot on the grid - first player's dots will be blue and the second player's (or computer's) will be red. The winner is the first to have four dots that can be joined by straight lines to form a square.

Squares can be any size and can be tilted.
 If you are not using the interactivity, you may like to print off some dotty paper.