2013-11-07
Explanation
The Joke
The first panel shows a quote often attributed to Albert Einstein: "If you can't explain it to your grandmother, you don't understand it." A student watches as his grandmother writes complex vector calculus equations on a chalkboard (what appears to be a cross product and dot product with unit vectors).
In the second panel, the student is in bed with his girlfriend, who asks "What's the matter, babe?" He replies dejectedly: "My professor said I don't understand cunnilingus."
The Humor
The joke works by taking the Einstein quote and applying it in an unexpected direction. The quote is normally about physics or academic subjects -- you should be able to explain complex topics in simple terms. But the comic flips it: the student's grandmother is the one who understands the complex math (she is writing equations on a board), and separately, his professor has told him he does not understand cunnilingus (oral sex). The implication of the Einstein quote is that if the professor says he doesn't understand it, he should try explaining it to his grandmother -- which is obviously an absurd and mortifying prospect. The comedy comes from the collision of the wholesome academic aphorism with a deeply inappropriate application.
References
The quote "If you can't explain it simply, you don't understand it well enough" is widely attributed to Albert Einstein, though its exact origin is disputed. The equations on the board appear to involve vector cross products and dot products with unit normal vectors, standard notation in vector calculus and physics.