What do you get when you cross an XXXX with a YYYY” is a common opening to a joke, leading the listener to prepare for the expected joke template and punchline. A rhetorical question is a question asked or stated to make a point, and not an actual inquiry with an expectation of an answer. The joke is that that sentence is a rhetorical question. So the answer is you don’t really get a joke, you just get a meaningless rhetorical question. Which is sort of funny, which actually does make it a joke
