What is "silly string"

n. Colored foam that streams out of a spray can, as a party favor; a ropey emission of this foam.

Silly String (generically known as aerosol string) is a toy of flexible, sometimes brightly colored, plastic string propelled as a stream of liquid from an aerosol can. The solvent in the string quickly evaporates in mid-air, creating a continuous strand. Silly String is often used during weddings, birthday parties, carnivals and other festive occasions, but it has also proven useful militarily to detect tripwires.