Silly String is a chil's toy: a flexible, brightly-colored plastic string, which is shot as a stream of liquid from an aerosol can. The string sets quickly in mid-air, allowing one to shoot a continuous strand of it. Silly String is popular for usage during weddings, birthday parties, school carnivals and other festive occasions. Often used to check for tripwires or bombs. The material is a polymer-based resin which is propelled by the chlorofluorocarbon Freon-12, which is one of a group of compounds implicated in the depletion of the ozone layer. Within a year after Silly String was introduced, the US banned Freon 11 and 12 because they allegedly harmed the ozone layer. The formulation was immediately switched by Whamo and others to Freon formulations which were allowed. For this reason, the product has been banned in Canada.