I'm not sure how this works, but it is interesting:
Bench'Mark is a project inspired from street culture and street art. It is a piece of public seating, a bench on which people are invited to leave their mark. The users can make graffiti on the bench using their finger. The written or drawn mark will randomly appear on the surface of the bench.The marks float smoothly on the surface of the seat... until someone else uses the bench! When someone sits down the marks left travel towards the person who sat, bumping onto his/her bum seeking for their attention. The marks keep following the person's movements on the bench and in turn, people can read, play or brush away the marks.
Oh, great. Having your bum sexually harassed by graffiti. It's a piece being displayed at the Royal College of Art (I think -- the original link points to this catalog, but it isn't clear where exactly it is in the show) by Louisa Stathopoulou, a product designer at Mitsubishi.