money-and-happiness
Explanation
The Joke
An older man is lecturing a child about the saying "money can't buy you happiness." He declares this is "just a category error" and compares it to saying "gasoline won't build you a car" -- implying that money is a necessary fuel or input for happiness, not the thing itself. The child then asks the obvious follow-up question: "What DOES buy you happiness?" The man dismisses the question as "Nitpicking."
The Humor
The comic skewers a particular type of intellectual who loves to tear apart conventional wisdom with clever-sounding logical arguments but has nothing constructive to offer in its place. The man sounds impressively philosophical when he identifies a "category error" and produces a neat analogy, but when pressed on the actual substance of his argument -- what does produce happiness, if not money directly -- he has no answer and deflects by calling the question "nitpicking."
The irony is that calling a genuine, important follow-up question "nitpicking" is itself a category error of sorts. The child's question is the entire point of the discussion, not a trivial detail. The man has essentially performed the intellectual equivalent of a magic trick: he deconstructed one platitude with apparent sophistication, only to reveal he has nothing underneath. This is a recurring SMBC theme -- characters who are more interested in sounding clever than actually being wise.