Good comment. I sometimes describe borrowing as renting money, where interest is the rent. Its a mathematically interesting construct because, for one thing, the value of the thing you rented goes down with cumulative rent paid. It's kind of a degenerate money cycle if you loan money to someone who uses it to pay you interest on the loan. I suppose only taxes (serving as friction, removing energy) prevent this from being a kind of economic perpetual motion machine! I'd be stunned if this trick wasn't a) very old, and b) have about 1000 modern variations.