No one in the lab could quite believe what they were seeing. An experimental device, a humidity sensor, had started generating electrical signals. Fine, you might think – except that shouldn't have been possible. "For some reason, the student who was working on the device forgot to plug in the power," says Jun Yao at the University of Massachusetts Amherst. "That's the start of the story." Since that moment five years ago, Yao and his colleagues have been developing a technology that can harvest electricity from nothing but humid air: a concept known as hygroelectricity.