When dropping salt, we throw some over our shoulder, to remove the curse it can bring. In Leonardo da Vinci's painting The Last Supper, Judas has knocked over a salt cellar, a sign of bad luck.
Salt was once very valuable, so what better way to discourage waste than to promulgate the myth that spilling it will bring you bad luck? In order to get rid of the devil you invite in by spilling salt, you have to throw a bit over your left shoulder, where he is sitting, to blind him. Deliberate ritual waste thus becomes the way of atoning for accidental, occasional spillage.
-from an article in The Guardian on the Friday the 13th effect: why so many restaurants are missing a table 13.
Photo: TYO/George Zorbas