explain xkcd: It's 'cause you're dumb.
'Undeclared' is sometimes called "General Studies".
Here's a YouTube video of "I Am the Very Model of A Modern Major's General"  for those who need to get the tune.
Here's is Tom Lehrer's Elements.
And here is Supercalifragilisticexpialidocious from Mary Poppins.
In panel 4, Methyl acetate is the solvent used to remove stamps from their envelope.
In panel 8, underwater basket weaving is a commonly used metaphor for any college major that is easy or worthless.
In panel 25, supermoon and zodiac are terms invented not by astronomers, but rather by early astrologists.
In panel 26, agronomy is the science of using plants as food or fuel, while the unrelated agoraphobia is the fear of wide open spaces.
And Sophie's Choice is any dilemma where choosing one cherished person or thing over the other will result in the death or destruction of the other, derived from the theme of the novel, and has also been turned into a romantic drama film.
People have done renditions of this song. See 
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Panel 1's cueball is in the same pose as Rodin's "The Thinker"
Panel 4 background is the periodic table of elements.
Panel 5, Fowler's Toad emits a noxious secretion that irritates skin and mucous membranes (it was previously thought to cause warts)
Panel 6, Psychology = a serial killer with a chainsaw, Sociology = hobo; Social Psych = hobo serial killer with chainsaw.
Panel 15, LISP, Scheme, and other computer languages with an excess of parentheses.
Panel 16, biohazard symbol
Panel 19, bongos were played by Richard Feynman
Panel 27, fear of snakes, study of reptiles
Panel 28, a picture of a stomach, pun on "stomach" being slang for "tolerate"
Panel 30, words in all lowercase like e.e.cummings
-- 126.96.36.199 22:04, 7 December 2012
In panel 22 (History), what's the theme connecting the years 1935, 1969, and 1991?
Wwoods (talk) 15:40, 21 May 2013 (UTC)
This explanation is very small for that big comic. I am starting to add the transcript and after that I will do more investigations to that opera. This should be the key to explain all the panels.--Dgbrt
) 19:13, 21 May 2013 (UTC)