It takes a lot of flip-flops to build a playground. That's one of those sentences I never expected to write, along the lines of, "It takes a lot of goldfish to fly an airplane." But yes, your standard informal footwear can be recycled into plastic lumber — and if you're sole inclined, you can do your part at any Old Navy store. Through May 21, just dump 'em in the handy bin, and a company from Trenton, N.J., will take it from there. TerraCycle Inc., a humble ecological empire built around a product called worm poop, will use them to create four playgrounds that it will donate to as-yet-unselected lucky locations across the country.