TANSTAAFL

"There ain't no such thing as a free lunch" (alternatively, "There is no such thing as a free lunch" or other variants) is a popular adage communicating the idea that it is impossible to get something for nothing. The acronyms TANSTAAFL, TINSTAAFL, and TNSTAAFL, are also used. Uses of the phrase dating back to the 1930s and 1940s have been found, but the phrase's first appearance is unknown. The "free lunch" in the saying refers to the nineteenth-century practice in American bars of offering a "free lunch" in order to entice drinking customers.

The phrase and the acronym are central to Robert Heinlein's 1966 science-fiction novel The Moon Is a Harsh Mistress, which helped popularize it. The free-market economist Milton Friedman also popularized the phrase by using it as the title of a 1975 book, and it is used in economics literature to describe opportunity cost. Campbell McConnell writes that the idea is "at the core of economics".

The "free lunch" refers to the once-common tradition of saloons in the United States providing a "free" lunch to patrons who had purchased at least one drink. Many foods on offer were high in salt (e.g., ham, cheese, and salted crackers), so those who ate them ended up buying a lot of beer. Rudyard Kipling, writing in 1891, noted how he

...came upon a bar-room full of bad Salon pictures, in which men with hats on the backs of their heads were wolfing food from a counter. It was the institution of the "free lunch" I had struck. You paid for a drink and got as much as you wanted to eat. For something less than a rupee a day a man can feed himself sumptuously in San Francisco, even though he be a bankrupt. Remember this if ever you are stranded in these parts.

TANSTAAFL, on the other hand, indicates an acknowledgement that in reality a person or a society cannot get "something for nothing". Even if something appears to be free, there is always a cost to the person or to society as a whole, although that may be a hidden cost or an externality. For example, as Heinlein has one of his characters point out, a bar offering a free lunch will likely charge more for its drinks.

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