Explain xkcd: It's 'cause you're dumb.
The comic lists a few 'factoids' about snakes, ranging from the mildly informative to the strictly tongue-in-cheek.
The first factoid references the hypothesis that snake venom was an evolutionary development of saliva that, over time, gradually became more toxic as snakes with saliva that was able to assist in subduing their prey possessed an evolutionary advantage. It then posits that the evolutionary branch that developed into venomous snakes began with a snake whose mutation gave him a mouth that was 'slightly more gross than usual'.
Additionally, the comic illustration accompanying the second factoid colors in a 'habitat range' on a map of South America that is snake-shaped, implying that when it states 'The longest snake is found in Brazil, Peru, and Chile' that this snake is so long that it literally stretches from Brazil, across part of Peru, into Chile, and that the 'habitat' shaded on the map is, in fact, this mammoth snake's silhouette. The age, length and location of the snake are so exaggerated that they are obviously untrue, but may be a reference to the Green Anaconda, one of the worlds largest snakes, which inhabits this region.
The final factoid is entirely tongue-in-cheek. Many factoids come in the form "If you laid all the X end to end, Y" would occur (E.g. "If you laid all the veins and arteries in the human body end-to-end, they would stretch 60,000 miles"). The Y portion of the factoid is supposed to be surprising, therefore it is ironic that the factoid in the comic, "If you laid all the bones in a snake end to end, you would have a snake.", is obvious and not at all exciting. Clealy, you would not have an entire snake, literally, but you would have a skeleton that was recognizably that of a snake and could reasonably be referred to as 'a snake'.
The title text presents the amusing idea that 'snakes' as we know them are not, in fact, a suborder of reptiles but are instead human digestive tracts that, rather than being a system of organs, are creatures capable of escaping from their 'host' human and living independently. The idea seems to follow from the superficial resemblance between snakes and the human digestive tract as long, roughly tubular collections of animal matter, which can process the food entering the top end, and getting rid of the waste in the other end.
- Randall had previously posted an incorrect map, that included the snake's habitat in Bolivia instead of Peru. 
- Snake Facts:
- Snake venom evolved from saliva, which means it all started with a snake whose mouth was slightly more gross than usual.
- [Picture of a snake below the text above]
- Snake: Hi guys!
- Off-panel voice: Eww, it's Frank.
- [Map of South America with gray shade in the form of a snake. Text to the left of it]
- The world's longest snake is found in Brazil, Peru and Chile. It is believed to be over 60 years old.
- [Picture of a snake skeleton between the first and the second of the lines below]
- If you laid all the bones in a snake end-to-end,
- you would have a snake
Given the habitat listed for the second factoid, it is likely the comic is referring to the Green Anaconda (Eunectes murinus).
- The Green Anaconda's habitat range includes Colombia, Venezuela, the Guianas, Ecuador, Bolivia, Brazil, Peru, the island of Trinidad, and Paraguay.
- The Green Anaconda is one of the longest snakes in the world reaching more than 6.6 m long.
- Anacondas generally do not live beyond 20 years in captivity, and likely less in the wild.
- Since anacondas are reported to continue growing throughout their lives, a 60 year old specimen would likely be the longest snake in the world.
- The Reticulated Python (Python reticulatus) is recognized as the longest, but not heaviest, snake and grow to more than 6.95 m.
- The Reticulated Python's habitat is in Southeast Asia.
The human digestive tract is essentially a hole that runs through the body, closed off most of the time only by sphincters, thus digestion can be said to take place outside the human body. Nutrients are absorbed across membranes via osmosis, active transport, and diffusion.
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I thought the Worlds longest snake was so long that it took up enough space to be in Brazil, Peru, and Chile at the same time.~~
Just a couple of thoughts: How big was the person whose digestive tract became the longest snake in the world? Also, does the grosser end of the digestive tract develop into the head of a venomous snake? 22.214.171.124 06:57, 23 July 2014 (UTC)
- Not a particularly important part of answering your question, but... which end of the digestive tract are you thinking is the "grosser end"? I could imagine arguments for both. Brettpeirce (talk) 12:30, 24 July 2014 (UTC)
The snake in the map shades Chile, BOLIVIA and Brazil, not Peru. 126.96.36.199 08:25, 23 July 2014 (UTC)
- Randall has corrected the map on xkcd... Someone should upload the updated version. 188.8.131.52 09:48, 23 July 2014 (UTC)
- Updated the image to match the one on xkcd.com. Nialpxe (talk) 10:00, 23 July 2014 (UTC)
Sorry, the shading (unless randell's updated after reading this) does track chili, up the Pacific Coast, across the border with Peru and veers east into Brazil.
I did wonder if 'the World's longest snake' was a reference to the south American highway, part of the pan American highway. Parts were completed in the 1950's making it 'over 60years old', and does track chili as per the 'snakes' body into Peru but the brazilian section is connected elsewhere, neatly crushing my wild theory. :-( 184.108.40.206 09:39, 23 July 2014 (UTC)
The first factoid contains a common misconception about evolution, namely that species evolve only in small steps. It's entirely possible that a small mutation caused a protein that appeared in snakes' saliva to suddenly be very poisonous to the snake's prey or enemies. --220.127.116.11 11:13, 23 July 2014 (UTC)
- It is possible, but unlikely. Snake venom is not a single chemical, but a mixture of several enzymes and toxins. There is a lot of variation in protein structure and enzymatic properties of the constituents of different snakes' venom, which suggests a gradual shift from one or two simpler lytic enzymes to a complex mixture. Each protein could have mutated separately, but the composition of the venom of each species almost certainly developed over a prolonged period of time. 18.104.22.168 11:34, 23 July 2014 (UTC)
- I don't think the implication was that every venom evolution happened in one fail swoop. I believe he was pointing out that instead of people thinking that venom evolution started with bad breath (minuscule unnoticeable changes) more likely started with a reasonably poisonous mutation that actually benefited a snakes survival.--Bmmarti3 (talk) 12:46, 23 July 2014 (UTC)
- Exactly what Bmmarti3 said. "the composition of the venom of each species" You mean of today's snakes. This is what we would expect for animals that have been around for almost 100M years. The venom of the first poisonous snakes was certainly weaker (and probably only worked on specific targets), but that doesn't mean it was weak. --22.214.171.124 13:05, 23 July 2014 (UTC)
Is it possible that the alt-text is a reference to Cyrano de Bergeracs "L’Autre monde ou les états et empires de la Lune" ("Comical History of the States and Empires of the Moon")? I don't have a proper translation, but there's a dialogue that says that every man is born with a snake inside his belly as a punishment by god.--126.96.36.199 15:35, 23 July 2014 (UTC)
- Is this possibly a reference to the Goa'ould from "Stargate"? 188.8.131.52 (talk) (please sign your comments with ~~~~)
That ive seen, Randall is usually very precise in his work. getting a wrong breed for 'longest snake' seems odd to me. Could the 'longest snake' be referring to either a 'longest lived' specimen, or a geographical feature named for a snake? Or perhaps an extinct species? 184.108.40.206 15:51, 23 July 2014 (UTC)
- I think he's referring to an actual specimen that is pictured. 220.127.116.11 18:12, 23 July 2014 (UTC)
I second the suggestion that this is not actually referring to a species of snake, or a particular specimen even.... It seems odd that Randall would post inaccurate information about snakes for no apparent reason. Perhaps this is a reference to something else entirely... previous dictators? Where in the World is Carmen San Diego? N'Sync reunion concerts? 18.104.22.168 (talk) (please sign your comments with ~~~~)
- Agreed. Anaconda's are not found in Chile either... though Google says that the Anaconda Copper Company owned the largest Copper Mine in the world, located in Chile. I can't connect that factoid with the "more than 60 years" part, though. 22.214.171.124 22:18, 23 July 2014 (UTC)
- anaconda copper was founded in 1881, making it 'more than60 years old' - not a great connection in my book. 126.96.36.199 15:00, 24 July 2014 (UTC)
- Anaconda Cooper got out of Chile in the 1960's, when cooper was 'chilenized' and then estatized in the 1970's.
--188.8.131.52 18:23, 24 July 2014 (UTC)
I agree that some people are taking this too literally. Imagine if you approached the situation in reverse, accepting the factoids about a word that you are unfamiliar with. Our digestive tract swallows food, processes it, and evacuates the waste through a flexible wavy tube with several independent parts. The skeleton of the snake looks like a wavy line of individual segments, maybe implying where the name came from. The shaded area on the map looks pretty spot on with the location of the Andes mountain range (the longest continental mountain range on Earth and definitely believed to be over 60 years old). The top factoid is about the animal we have named after the concept of a snake as a wavy not straight staying thing. I like this idea and think I shall keep it. Thank you everyone for your help in offering inadequate information, and while that may seem sarcastic, it is not. I regularly use incorrect examples to get my mind on the correct path to good ones.184.108.40.206 06:32, 24 July 2014 (UTC)
- Created an account to accept potential wrath. JovialRoger (talk) 06:38, 24 July 2014 (UTC)
- The Andes extend from north to south through seven South American countries: Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina. The peak is in Argentina. Randall would probably take the opportunity to make the snake longer, if he had meant the Andes. Plus, no Bolivia/Peru map issue. 220.127.116.11 07:42, 24 July 2014 (UTC)
There's an obvious reference to Ze Frank's True Facts series here, both in the amount of truth the facts have, and the use of the name Frank.
18.104.22.168 07:49, 24 July 2014 (UTC)jivadent
Is the 'wikipedia says' at the start of every sentence necessary?22.214.171.124 08:48, 24 July 2014 (UTC)
Possible reference to a YouTube video uploaded 14 Feb 2014 explaining a partial solution to the Erdos Discrepancy Problem, which includes: "Two steps in front of you is a nest of angry snakes. And not just any old angry snakes, but like, snakes with really bad breath and stuff".  --DivePeak (talk) 08:22, 25 July 2014 (UTC)
I immediately thought of a currency snake, an agreement between two or more countries to keep variation in the value of their currencies in a certain range. Thought the three countries mentioned could have made such an agreement ca. 60 years ago. Admittedly I find no evidence of it whatsoever.21:45, 25 July 2014 (UTC)~