qed
Explanation
The Joke
A student asks a professor why mathematical proofs end with "those little dots" (referring to the QED symbol or the three-dot "therefore" symbol). The professor explains that it dates back to a time when "you had to convince the other math people you weren't doing black magic" — it stands for a Latin phrase that basically means "see, I'm not a witch, this is just proof."
The student then asks the professor to "solve the same variable for two things at step 6." In the final panel, the student is shown hissing "SSSSSSSSS" — reacting like a vampire or demon exposed to holy water, implying the student actually is some kind of dark magical creature threatened by legitimate mathematics.
The Humor
The comic sets up a fake historical explanation for the QED notation (which actually stands for "quod erat demonstrandum" — "that which was to be demonstrated"). The professor's joke version — that it was to prove you weren't doing witchcraft — seems like a throwaway gag. But then the punchline reveals that the student genuinely recoils from mathematical proof like a supernatural creature recoils from sacred objects.
The humor inverts the joke: what seemed like a silly historical anecdote turns out to be literally true within the comic's universe. There really are people (or creatures) for whom rigorous mathematical proof is an existential threat. It's also a commentary on how some students seem almost allergic to formal mathematical reasoning.
Votey
The red-button panel typically adds an additional punchline that extends or undercuts the comic's premise.