
“An alleged scientific discovery has no merit unless it can be explained to a barmaid.”
As quoted in Einstein: The Man and His Achievement (1973) by G. J. Whitrow, p. 42
Variants:
If you can't explain your physics to a barmaid it is probably not very good physics.
As quoted in Journal of Advertising Research (March-April 1998)
A theory that you can't explain to a bartender is probably no damn good.
As quoted in The Language of God (2006) by Francis Collins, p. 60