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Kermit the Frog famously said, "It's not easy being green." Easy or not, though, many U.S. cities are working hard to be as green as possible. Recycling, public transportation and alternative energy sources like bio-fuels, solar and wind power are among some of the indicators used to determine how green a city is. Other factors include parks and trees, consumption of local foodstuffs, water quality and sustainable building techniques. Cities doing the best job of meeting these criteria are the greenest cities in the United States.