The pet rock, a popular gift, came with a carrying case and a care and training instruction book. AP/Wide World Photos. Reproduced by permission.

The pet rock was one of the biggest gift crazes in American history. Introduced in time for the holiday gift buying season in 1975, the humble stones sold for two dollars apiece and came with a special "training manual." The man who "invented" the pet rock became an instant millionaire.

Pet rocks were the brainchild of Gary Dahl (1937?–), a California advertising man who came up with the concept while having dinner with some friends in April 1975. Dahl thought, What if there were a pet that did not require any care or feeding? He quickly set about writing his Pet Rock Training Manual. Soon after, he decided to market an actual pet rock as well. He bought a bunch of Rosarita beach stones...