Barmaid physics
A rendition of “barmaid physics” type explanations, wherein the guy tries to explain, at left, Ben Biddle’s 2013 “Innovation is like a Chemical Reaction” cocktail napkin idea, and at right, Roger Penrose’s 2004 conjecture of the second law, under the stipulation of a positive cosmological constant, as: “Entropy per baryon tends so increase relentlessly and stupendously with time” to a beautiful, albeit simple minded (see: Beckhap's law) female bar tender. [4]
In science, barmaid physics, aka "barmaid explanations", refers to the oft-cited premise that, in physics, one doesn’t full understand a phenomena, theory, concept, principle, or law, etc., completely, until one can explain it to a barmaid or child, e.g. in simple words, or on a cocktail napkin.
Anecdotally, the origin of this simplification rule for difficult concepts is said to have occurred in a real situation in the working existence of either: James Maxwell, Charles Wilson, Ernest Rutherford, and or to Albert Einstein; albeit most oft-cited is Rutherford.

The following are quotes germane to the philosophy that if one cannot explain a phenomenon to a child than one does not understand the phenomena

“If you can’t make a physical model of it, you don’t understand it well enough.”
James Maxwell (or William Thomson) (c.1870), stated somewhere, in thereabout this form [?]

The following are variants of the quote attributed to cloud chamber inventor Charles Wilson:

“The laws of physics should be simple enough that a barmaid can grasp them.”
— Charles Wilson (c.1911) (Ѻ)

The following are variants of the quote attributed to alpha particle discoverer Rutherford:

“It should be possible to explain the laws of physics to a barmaid.”
Ernest Rutherford (c.1915) [5]

“If a piece of physics cannot be explained to a barmaid, then it is not a good piece of physics.”
— Ernest Rutherford (c.1915) (Ѻ)

“A good scientific theory should be explicable to a barmaid.”
— Ernest Rutherford (c.1915) (Ѻ)

“No physical theory is worth much if it cannot be explained to a barmaid.”
— Ernest Rutherford (c.1915) (Ѻ)

“A scientist who can’t explain his theories to a barmaid doesn’t really understand them.”
— Ernest Rutherford (c.1915) (Ѻ)

“An alleged scientific discovery has no merit unless it can be explained to a barmaid.”
— Ernest Rutherford (c.1915) (Ѻ)

The following are variants of the quote attributed to Einstein:

“All physical theories, their mathematical expressions apart, ought to lend themselves to so simple a description that even a child could understand them.”
Albert Einstein (c.1930), comment to Louis de Broglie [5]

“If you can't explain it simply, you don't understand it well enough.”
— Albert Einstein (1947), supposedly derived (Ѻ) from Rutherford “barmaid” comment [6]

The following are related quotes:

“One major difference between the ‘games’ played by theoretical physicists and those played by pure mathematicians is that, aside from meeting the demands of internal consistency and mathematical rigor, a physical model must also meet the inflexible boundary condition of agreeing with physical reality.”
— James Cushing (date); compare Gibbs on mathematicians vs. physicists [7]

1. Clark, Ronald W. (1984). Einstein: the Life and Times (pg. 418). HarperCollins.
2. Kami, MIchale J. and Martz, W.F. (1998). Management Golf: What’s Your Handicap? (pg. 73). CRC Press.
3. Kennedy, Robert E. (2012). A Student’s Guide to Einstein’s Major Papers (pg. 1). Oxford University Press.
4. Thims, Libb. (2014-17/18). Chemical Thermodynamics: with Applications in the Humanities (pdf) (Biddle, pg. 14). Publisher.

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