The next time you are enjoying a Guinness at your local bar, along with containing a cold beer your pint glass may also reveal a QR code. Unreadable when empty or filled with a standard, lighter-colored beer, the QR code becomes active when filled with Guinness. (Or any stout, porter, or dark beer, I'm going to assume. If it only worked with Guinness I would be really impressed with the engineering.)
The QR code unlocks the ability to share your Guinness pint-drinking status on Facebook, Twitter and Foursquare, possibly making your friends instantly thirsty. Who knows, they may even join you for a pint.