The exact value of a positive tweet about a brand has been officially pinpointed: It's worth a bag of new Special K chips. At least, that's the going rate at the new Kellogg's Tweet Shop in London—a pop-up store that's giving away bags of the new product in exchange for a little love on Twitter. Visitors to the store can pick out one of three prefab tweets to send—or they can come up with their own,  which might be less informational but could seem more authentic, too. They get a free snack in return. "The value of positive endorsements on social media sites is beyond compare, so we're excited to be the first company to literally use social currency instead of financial currency to launch this new product in our bespoke Special K shop," says brand manager Sarah Case.  British agency Slice worked on the campaign, which is a tasty development indeed. Photo: Ken Lennox.