a-frankenstein
Explanation
The Joke
Two people in Halloween costumes are talking. One, dressed as a monster, says they are going as Frankenstein. The other corrects them with the classic pedantic objection: "It's Frankenstein's MONSTER. The creature never gets a name in the book. His creator is Frankenstein, and he goes by 'Adam' or 'the monster.'" Rather than being embarrassed, the first person fires back: "You named YOUR costume after thinking up that rant, didn't you?" -- implying the pedant chose their own costume specifically as bait for this lecture. The pedant confirms this by saying "I told you it was a trick."
The final exchange is a double pun: the "trick" refers both to the Halloween tradition of "trick or treat" and to the social trick of deliberately setting up a situation where they could deliver their prepared correction.
The Humor
The comic takes the famously overused "actually, it's Frankenstein's monster" correction -- one of the most well-known pieces of literary pedantry -- and turns it on its head. Instead of the pedant winning the exchange, they are exposed as having engineered the entire situation just to deliver their rehearsed lecture, which is arguably more embarrassing than the original "mistake." The Halloween setting allows for the clever "trick" double meaning, tying the social manipulation to the holiday theme. It satirizes the type of person who derives more pleasure from correcting others than from genuine conversation.
References
Mary Shelley's novel "Frankenstein; or, The Modern Prometheus" (1818) is the source of the perennial debate about whether the creature should be called "Frankenstein" or "Frankenstein's monster." In the novel, the creature is indeed never formally named by his creator, Victor Frankenstein, though the creature refers to himself as "the Adam of your labors." The correction has become a cultural cliche, often deployed as a marker of literary knowledge.