You can't be arrested for vandalising your neighbourhood when technically you're cleaning it up -- right? That's the general idea behing the Grime Writer pen. It's an oversized felt tip marker that you crack open and fill with soapy water, instead of ink. So when you tag the side of a building, technically you're just removing years of grime and filth.
It turns anyone into a Robin Hood-like character who walks the line between doing good and bad. Because scrawling giant profanities on overpasses and water towers, even though you're technically doing your city a service, will probably still be looked down upon. Even worse, there's no reason you can't just fill this $US12 marker with permanent ink and turn it back into a tool of mischief.