TANSTAAFL is the acronym for "There ain't no such thing as a free lunch," a universal truth having broad and deep explanatory power in science and daily life.
The expression originated from the practice of saloons offering "free lunch" if you buy their overpriced drinks. Science fiction master Robert Heinlein introduced me to TANSTAAFL in The Moon is a Harsh Mistress, his 1966 classic, in which a character warns of the hidden cost of a free lunch.
The universality of the fact that you can't get something for nothing has found application in sciences as diverse as physics (Laws of Thermodynamics) and economics, where Milton Friedman used a grammatically upgraded variant as title of his 1975 book There's No Such Thing as a Free Lunch. Physicists are clearly on board with TANSTAAFL, less so many political economists in their smoke and mirrors world.
My students hear a lot about TANSTAAFL, from the biological costs of the peacock's tail, to our nervous system that distorts physical reality to emphasize changes in time and space. When the final tally is made, peahens cast their ballot for the sexually exquisite plumage of the peacock and its associated vigor, and it is more adaptive for humans to detect critical sensory events than to be high fidelity light and sound meters. In such cases, lunch is not free but comes at reasonable cost as determined by the grim but honest accounting of natural selection, a process without hand-waving and incantation.