For years now—a decade, at least—the Complete and Total Loser has kept a spare twenty-dollar bill in a little container attached to his key chain, and he's taped a piece of paper with his phone numbers on the outside. The idea of this is that if he loses his keys, whoever finds them will call the number. They can keep the $20 as a reward.
He has yet to lose his keys, but there have been a few times when he's gone to a store and realized he's forgotten his wallet yet he's managed to buy enough of what he needs with the $20, make the trip less than a total loss.
The Loser shows this to friends and they say, "Huh. Good idea." None of them, as far as he knows, has ever done it themselves.
Now, the idea has been marketed as part of other things to attach to a key chain, and you can buy it at Staples for over twice the Loser paid for his at a camping supply store.
It's a pity you can't copyright an idea.