Why a cleaner sidewalk will make you think of pizza.

We here at WalletPop like to think we've seen it all when it comes to advertising mediums, but Domino's served up a new one recently: sidewalk grime. More accurately, the lack of sidewalk grime.

If that sounds puzzling, read on. To promote its new American Legends line of pizzas featuring regional toppings, Domino's hired a company called Green Graffiti to power-wash its logo along with the phrase "American Legends" on sidewalks in Los Angeles, Philadelphia and New York City.

Green Graffiti, a Dutch firm, has done similar advertisements in numerous European cities. The "green" in their name refers to the fact that the company uses no paper or ink the way traditional advertising does. But... what about all the water it takes to etch ads onto dirty sidewalks? Green Graffiti donates to a program in South America that provides clean drinking water to make up for all the H2O they spill on the pavement.

Dominos paid to have more than 200 of the logos created. (In case you're curious, they're made by plunking an oversized stencil down onto the concrete and then hitting it with a hose.) The ads have an interactive element, too. The first 250 people to email photos of themselves next to one of the sidewalk ads will receive $15 Domino's gift cards. Domino's is also posting pictures of the design-discoverers on Flickr, which will ensure that this campaign stays in people's minds long after those fresh-scrubbed logos fade back to grunge again.