What is arguably the weirdest invention of all time to make millions of dollars wasn't really an invention at all: The Pet Rock.
In case you don't recall this rocky fiasco, Gary Dahl, an advertising executive from California, figured out a way to decorate and market rocks as pets. It might not sound like much, but it spawned a fad that swept the U.S. like a runaway rockslide.
Dahl purchased ordinary gray pebbles from a construction supplier and sold them as pets. Some models sported painted faces, while others bore glued-on googly eyes on their stony countenances.
In a testament to marketing, and to P.T. Barnum, Dahl billed the pet rock as the perfect pet, one that never needed to be fed or cleaned up after. As outlined in a humorous manual included with the "pet," owners could talk to it, name it or teach it to do simple tricks. Many owners painted them or found other ways to personalize their rocks.
The Pet Rock debuted in 1975 at $3.95 -- about $16 in the current economy. In just six months, Dahl sold more than 5 million pet rocks, raking in a profit equivalent to $56 million in 2011 dollars, in large part because of his lack of overhead: Buying the rocks and delivering them probably cost only 95 cents apiece [source: PetsDo].
Pet Rocks have staged a comeback on the Internet in recent years. You can still buy them online, though many are fancier than Dahl's originals.
Does that qualify as progress?
See our other countdowns of groundbreaking inventions throughout history.