On a balmy winter day, Irfanullah Wahid and his cousin Faisal Asadullah amble through a maze of carts in Karachi’s Shireen Jinnah neighbourhood. They are young – only 11 and 10 years old respectively – and the white bags they carry are almost as tall as they are. They laugh and joke, but their eyes are peeled. Every few steps, they pause, bend, pick something up off the street and slip it into the bags. Wahid collects only metal cans. Asadullah sticks to thick plastics. Asadullah stoops to pull...