Before toy stores were stocked with pet plants, virtual pets and pillow pets, an advertising copywriter called Gary Ross Dahl was dreaming up the Pet Rock, a 1970s novelty toy craze since called a “ridiculously successful marketing scheme.” It was meant as a joke — a “pet” for people who didn’t want to care for one. But it hit the market at the right time. The Vietnam War had ended. Watergate was beginning. “There was a whole lot of bad news going on,” he told the Houston Chronicle in 1999.