Linguistics jokes are great. Here is one that was posted on Facebook by linguist Jonah Katz on September 26, 2024 (original source unknown).
The humorous effect here has many layers, and draws in part on an ambiguity that occurs in written English between a polar question containing “or” and an alternative question. A polar question is one that asks for a “yes” or “no” answer from the addressee, such as “Do you like cats?”. The question in panel 2 can have this reading, and happens to contain “or” – if the speaker on the left does either of the two things, the answer would be “yes”, otherwise the answer would be “no”. An alternative question also contains “or”, but on this reading, the question does not ask for a “yes” or “no” answer; rather, the answer to an alternative question is one of the two things connected by “or” (the disjuncts). Additionally, we usually think of alternative questions as presupposing that one of the disjuncts is true.
In this meme, the polar question interpretation is likely intended by the speaker on the right – an answer of “yes” to the question as a whole would provide an explanation for why the speaker on the left’s teeth are turning yellow. (As an aside: it’s interesting that the polar question interpretation is the most natural one here, because in many contexts this is not the case.) And unless the speakers know each other quite well, it might be presumptuous for the speaker on the right to use a question form (i.e., an alternative question) that presupposes that the speaker on the left either smokes or drinks coffee. But the speaker on the left has interpreted the question as an alternative question, and answers with one of the disjuncts: He drinks coffee. So part of the humorous effect here is that the speaker on the left has mis-interpreted the speaker on the right’s question.
This ambiguity only occurs in written English. In spoken English, prosody disambiguates between the two question types. Polar questions in (many varieties of) English occur with a final rise intonation, whereas alternative questions have a rise on the first disjunct (actually, on all the non-final disjuncts if there are more than two things being connected by “or”), and a fall on the final disjunct. This means that in a real life conversation, it would be very unlikely that the speaker on the left would mis-interpret the question in panel 2, since the speaker on the right would have used one of the intonation contours, thereby disambiguating their question.
The humorous effect of the meme further draws on the fact that there are actually three readings for the question in panel 2 – there are in fact two alternative question readings, depending on exactly how the prosodic contour is pronounced, i.e., what is in focus. This in turn has an effect on how we interpret what the disjuncts are. On one structure (a): Do you [[smoke] or [drink coffee]], the question is asking whether the speaker on the left smokes or whether he drinks coffee. With the other structure (b): Do you [[smoke or drink] coffee], the question is asking whether the speaker on the left smokes coffee or drinks it. The (b) interpretation is non-sensical given world knowledge (we don’t smoke coffee), but panel 3 makes clear that this is in fact the interpretation that the speaker on the left has given the question, adding a further layer to the humour conveyed here. His use of the pronoun “it” in panel 3 is crucial here (as pointed out by Seth Cable in a comment on the aforementioned Facebook post). The use of “it” seems only compatible with the (b)-reading here – if the speaker on the left had said “I drink coffee”, this would amount to an answer to the (a)-reading of the question, and would not be construed as a joke.