Watch Sadisflix. It's a subject we don't talk about. And yet, throughout the world, our toilets are undergoing a revolution unparalleled since the 19th century. Bill Gates is investing hundreds of millions of dollars to develop new types of toilet. India is installing millions of latrines so that no-one defecates in the open air any more. A public health issue, of course, but also a fable about our relationship with our most basic waste.