To knit or crochet something and leave it out in the public space is a relatively new phenomenon. The act has been called yarn graffiti, yarnstorming og yarnbombing.
In October 2011 I decided to yarnstorm to celebrate Breast Cancer Awareness month and help raise awareness about breast cancer.

This is a pattern for a Pink Breast, very suitable for yarnstorming.

Note: The pattern is written with US crochet terms so if you crochet the UK way, be aware that a single crochet in US terms is a double crochet in UK terms.