A heat-sensing film could let robots detect when humans are around, like pit vipers hunting out warm-blooded prey. The flexible, transparent coating is made of pectin, a low-cost plant material used to set jam. Unlike conventional electronics, it relies on currents of ions rather than electrons to detect temperature variations – just like natural membranes used by the snakes. The film can sense temperature changes as small as 10 millikelvin, which is twice as sensitive as human skin. It can detect a warm body the size of a rabbit from a metre away, something the researchers tested by microwaving a teddy bear and setting it at different distances from the film.