Magic Tricks

The Magic Square is an ancient mathematical curiosity, said to have been appeared in Chinese around 2800 BCE. Better known is the 4x4 magic square in Durer's engraving of 1514.
The well known modification included here, making it "automatic", means that the square can mostly be completed in advance, avoiding the boredom of watching while the magician does the mental arithmetic for the whole thing.
There are a number of ways to make it magic, and the best one I know is the "NFA Magic Square" by Mark Elsdon, where the magician arranges to get a peek of the spectator's number rather than lamely asking for it.