In France, the holiday is called Poisson d'Avril (April's Fish).French children in this day stick paper fish on the backs of their friends. When an object detects irony joke, prankster yells: "Poisson d'Avril"! Incidentally, one of the nicknames of Napoleon was nicknamed "Poisson d'Avril", due to the fact that he married Marie-Louise of Austria was the 1st of April