rock
Explanation
The Joke
Two cavemen are having a conversation. One explains, "This is rock. Is called 'rock' because last week, Pag called it 'rock.'" The other caveman responds with exaggerated fascination: "Is SO interesting!" The caption below reads: "In the early days, etymology was much easier."
The Humor
The joke plays on the nature of etymology -- the study of the origins and history of words. In modern language, word origins are incredibly complex, tracing through centuries of linguistic evolution, borrowings from other languages, sound shifts, and cultural changes. Etymologists spend entire careers tracing how a single word evolved. But the comic imagines a time when language was brand new and words were just being invented on the spot by specific individuals, making the entire field of etymology trivially simple: the word "rock" exists because one specific guy named Pag decided to call it that last week. The humor is also enhanced by the second caveman finding this utterly banal fact "SO interesting," parodying the enthusiasm that linguistics nerds bring to genuinely complex etymological discoveries.