Take a parking spot, cover it in grass, add a bench and a single tree. Then watch delighted urbanites settle in for a respite while you feed the meter. The San Francisco design group Rebar originally used this formula in 2005, morphing 200 square feet of street space into an unlikely park.That original "parking intervention" has spawned Park(ing) Day, an annual seizure of asphalt for the public commons. On September 21, 2007, roughly 35,000 square feet of parking spots, in 47 cities-from Paris to Wilkes-Barre, Pennsylvania-became parkland, with new variations including croquet games, chicken coops, and water gardens.Rebar and Public Architecture led the charge in San Francisco, where private vehicles occupy 70 percent of public space. They seized the mayor's personal spot with a pedal-powered "mobile park" (a wheeled platform with a park on it) fashioned by a sculptor named Reuben Margolin. "This way we can deploy open space whenever and wherever it is needed," said Rebar's Blaine Merker. "As far as I know, we are the only park in the city that's in motion."LEARN MORE parkingday.org