Rocks, once a hindrance since they reduced arable land, have become an asset. Pedrina Pereira and João Leite used them to build four ponds to collect rainwater in a farming community in Brazil’s semi-arid Northeast.

How should cities address the problem of waste? The most important thing is to set a clear objective: that the day will come when nothing will be sent to final disposal or incineration, says an international expert on the subject, retired British professor of environmental chemistry and toxicology Paul Connett, author of the book “The Zero Waste Solution.”