Last month, at a vast composting yard owned by a Northern California waste and recycling company, Recology, I watched a load of lawn and food scraps from San Francisco residents get fed into a sorting machine. A spinning cylinder resembling a supersized cheese grater sifted out tidbits like lime wedges and grass clippings and spit the chunkier items onto a platform, where a worker in a neon vest plucked out plastic bags and an aerosol can of glass cleaner--just a few of the hundreds of pieces of contraband that he'd cull that day. I asked if he ever let anything slip by. "Sometimes," he said with a sheepish smile. I later ran my hand through a ripened compost pile and felt little pieces of glass and plastic mixed in with the fertile humus.