In 1975, advertising executive Gary Dahl was sitting in a bar listening to friends complain about their pets. Dogs needed walking, cats scratched furniture, fish died if you forgot to feed them. Dahl joked that he had the perfect pet: a rock. It never needed food, wouldn't die, and required zero maintenance. What started as a gag became one of the most brilliantly absurd business ideas in history.

Dahl bought ordinary gray stones from a builder's supply store in Mexico for about a penny each. The genius was in the presentation. Each rock came in a cardboard carrying case designed like a pet carrier, complete with breathing holes and a bed of straw. But here's what really sold it: a 32-page training manual. The booklet taught owners how to make their pet "sit" (easy) and "stay" (very easy), plus commands like "attack" and even how to housebreak it. The manual's deadpan humor made the whole package feel like you were in on an elaborate joke.

Within six months, Dahl sold over 1.5 million Pet Rocks at $3.95 each, making him a millionaire. He appeared on "The Tonight Show," and the fad exploded across America during the 1975 Christmas season. By February 1976, it was over—everyone who wanted a Pet Rock had one. Dahl never trademarked the idea, so knockoffs flooded the market, but he'd already made his fortune. He later called it "one of the most ridiculously simple ideas ever."