Yarn bombing, aka yarnstorming, guerrilla knitting, or graffiti, is a form of street art that has been spreading across the globe over the last few years. Knitters and crocheters take their “matronly craft” to the streets covering urban objects with brightly coloured fuzzy goodness. Few objects are off limits and tree trunks, hydrants, mailboxes, park benches, public telephones, bicycles, public sculptures, and even cars and buses have been yarnstormed.
The movement began in 2005 with a single door knob cozy knitted by Magda Sayeg of Houston, Texas. This led to the formation of the knit graffiti collective called Knitta Please. Other collectives followed including YarnCore collective in Seattle, Knit the City in London, and Ladies Fancywork Society in Denver. There is even a book Yarn Bombing: The Art of Crochet and Knit Graffiti, 
While yarn bombing appears harmless enough, it is still technically illegal under vandalism and/or littering laws. “Yarn bombers say they rarely have run-ins with the law. And in the few instances when they are stopped … the police are more likely to laugh at them than issue a summons.” (Daily Art Fixx)