Explain xkcd: It's 'cause you're dumb.
This is a play on the traditional police officer strategy of "Good Cop, Bad Cop", in which two officers play different parts to get the suspect to give the required information. One is nice to the suspect and the other is mean to the suspect.
However, in this comic, they use the "Good Cop, Dadaist Cop" strategy in which one is nice to the suspect and the other is Dadaist, which is defined as (via the Free Dictionary) a European artistic and literary movement (1916-1923) that flouted conventional aesthetic and cultural values by producing works marked by nonsense, travesty, and incongruity.
So, the Dadaist cop is spouting nonsense attempting to get the suspect to give some information. Unfortunately, this probably is not going to work.
Mark Zuckerberg is the co-founder of Facebook; Church Latin (aka Ecclesiastical Latin) is a particular style of the Latin language used mainly by the Catholic Church. All that, however, is beside the point because Zuckerberg does not own a house (and thus does not have a mortgage), he rents. 
- [Two cops look through a window into an interrogation chamber holding a handcuffed suspect.]
- Good Cop: All right, let's try good cop, dadaist cop
- [Good Cop is seated in front of the suspect]
- Good Cop: Look, you're a good guy. We can work this out. Hey, lemme get us some coffee.
- [CHANGE PLACES]
- [Dadaist Cop holds up a document of indeterminate contents and threatens the suspect with it]
- Dadaist Cop: See this? It's Mark Zuckerberg's Mortgage. So why is it written in CHURCH LATIN?
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- [Dadaist cop physically rattles the suspect]
- Dadaist Cop: WHY ARE MY BONES SO SMALL?
- Suspect: What's WRONG with you!?
- Dadaist Cop: What's wrong with ART?
A comment from the blog that is quite on and off (you'll get the joke) the topic:
- I took a class in college, in French, and we studied (insofar as one can) dadaism, surrealism, and existentialism.
- One day, the girl next to me raised her hand and started out, “This is off the subject, but..”… Professor La Charité interrupted immediately, with, “It’s *never* off the subject. Continue.”
- We all felt we learned something that day. Giraffe. - E
Hope that explains some things. lcarsos (talk) 17:40, 20 August 2012 (UTC)
I think the above explanation is lacking. What's bones got to do with it? How about the language stuff? AND A Mexican bandit robbed a bank. The sheriff and his bilingual deputy captured him, and the sheriff, who couldn't speak Spanish, asked him where he'd hidden the money. "No se nada," said the bandit.
The sheriff put a gun to the bandit's head and said to his deputy: "Tell him, if he doesn't tell us where the money is, I'll blow his brains out."
Upon receiving the translation, the bandit became very animated. "Ya me acuerdo! Tienen que caminar tres cuadradas hasta ese gran arbol. Debajo del arbol, alli esta el dinero."
The sheriff leaned forward. "Yeah? Well..?"
The deputy replied: "He says he wants to die like a man." 188.8.131.52 (talk) (please sign your comments with ~~~~)
- It's Dadaism. It means unrelated random stuff. AND nice story =) 184.108.40.206 19:26, 29 January 2014 (UTC)
What's incomplete about this? 0100011101100001011011010110010101011010011011110110111001100101 (talk page) 05:37, 2 August 2014 (UTC)
- If you twisted my arm, I'd have to guess that my phone is ringing off the salmon. That's all I can remember at this indication, though. -- Brettpeirce (talk) 11:48, 13 August 2014 (UTC)
- This is why we can't have giraffes. 220.127.116.11 09:40, 4 September 2016 (UTC)
Has Mark Zuckerberg ever HAD a mortgage?18.104.22.168, 27 Dec 1014
Well interesting fact about ponytail then, her bones are small!
--Dalonacueball (talk) 12:48, 24 March 2015 (UTC) 13:35 3/24/2015
The implication, in the explanation text, that wealthy individuals, such as Zukerberg, wouldn't take a mortgage is flawed. Wealthy individuals usually do take out mortgages because they have the ability to generate a higher rate of return from the freed capital than the cost of the interest on the mortgage. Mountain Hikes (talk) 22:17, 29 January 2016 (UTC)