Punchline
Explanation
The Joke
A woman asks a man, "Do you think time is real?" He responds confidently that of course it is -- "The arrow of time is obvious in comics because they always start with a setup and end with a punchline." The next two panels show both characters simply standing there in silence, staring at each other, with no punchline delivered.
The comic is self-referential: the character claims that comics demonstrate the arrow of time by always ending with a punchline, but this comic deliberately withholds its punchline. The final two silent panels are the joke -- the absence of a punchline is itself the punchline.
The Humor
This is a meta-comic that plays with the form of the comic strip itself. By having the character assert that time's directionality is proven by the setup-to-punchline structure of comics, and then refusing to deliver a punchline, the comic undermines its own character's argument. If there is no punchline, does time still have direction? The empty panels create an awkward, expectation-defying silence that is funny precisely because of what is missing. It is a self-defeating prophecy -- the statement about punchlines becomes false by the comic's own structure, creating a paradox that is itself humorous. The concept of the "arrow of time" is a real physics concept relating to entropy and the irreversibility of time, which adds an intellectual layer to the gag.