Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic depicts an English professor tied to a chair by a former student who has "turned to evil" after failing the professor's class due to poor grammar. The villain devises a diabolical trap: he will send the professor a text message, and if the professor texts back, he will explode. Otherwise, he goes free. The professor is confused about why he would ever text back.

Then the text arrives: "Your going to die." The professor sees the incorrect use of "your" (instead of "you're") and is so compulsively driven to correct the grammatical error that he texts back the correction -- "you're!" -- and promptly explodes. The joke plays on the stereotype of grammar pedants who physically cannot let an error go uncorrected, even at the cost of their own lives. The villain's plan is brilliant precisely because he understands his former teacher's psychology: the one thing an English professor cannot resist is correcting bad grammar, even in a life-or-death situation.