He charged $3.95 for a stone with a training manual

The product spec fits on one line: a beach pebble, $3.95 in 1975, about twenty-four of today's dollars. Inside the box, a bed of wood shavings and a twenty-page training manual. The stone was bought by Gary Dahl, a freelance advertising copywriter in Los Gatos, California, from a sand and gravel company that brought them in from Rosarito beach, in Baja California, for less than a cent apiece. Smooth, grey, the size of an egg. No moving parts. No battery.

The stone travelled in a cardboard box with a handle, just like the ones for taking the cat to the vet, with ventilation holes cut into one side and a bed of wood shavings inside. The ventilation is, strictly speaking, unnecessary.

And then there was the manual. It was called "The Care and Training of Your Pet Rock" and it was written with the straight face of a serious breeding guide, with a chapter on obedience and a chapter on tricks. The trick buyers liked best was "play dead", which the rock performs with a reliability no dog can match.

The idea came out of a California bar in the spring of 1975, with friends of Dahl's complaining about their pets: the food, the hair, the vet, the chewed sofa. Dahl told them his pet was a rock and that it never gave him a moment's trouble. He went home and wrote the manual.

He launched it at the San Francisco gift show in August 1975. At the peak of the Christmas season he reckoned up to a hundred thousand rocks a day were going out. Within a few months he had sold between 1.3 and 1.5 million, at the ninety-five cents of profit each that he himself claimed, and had made himself a millionaire out of a material that cost him less than a cent.

Six months later the fad was dead. Dahl went on to try other notions — among them a Bicentennial rock with a flag painted on it — and none of them found a buyer. He died in 2015, aged seventy-eight, known for ever after for a beach pebble.

Fifty years on, the product has lasted better than anything else of its era. The rock your uncle bought in 1975 is still in a drawer, intact, without a single trip to the vet and having never once run off, and whoever gave it to him no longer remembers. The only thing that has been lost is the manual, and without the manual the rock no longer knows how to do anything.