Hip is a game invented by the great Martin Gardner in the 60s. Its name comes from its objective: to avoid squares. Gardner's pun - don't blame me!
Game version: Two players take it in turn placing pieces of their own colour on the grid. The loser is the first one to place a stone that makes the fourth corner of a square, be it parallel to the board edges or at an angle.
Puzzle version: Try to find a drawn position (playing the two colours in turns until every square is filled). To save wasting your time, this only works for 6x6 boards or smaller.