Hidden pint-glass QR code is only visible when filled with Guinness

A clever bit of advertising gimmickry from Guinness: these pint glasses bear QR codes than can't be read when the glass is empty, nor when it is filled with amber-colored beers. But when filled with black, murky Guinness, the revealed QR code can finally be scanned: "it tweets about your pint, updates your facebook status, [...]

Source: http://boingboing.net/2012/05/18/hidden-pint-glass-qr-code-is-o.html

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