Sam’s usual answer is, “Yeah, and telling that joke for the thousandth time won’t make it funny.” But if he’s in the right mood—and that’s most of the time nowadays; he’s not going to speculate why—he’ll answer, “How?”
Dean puts on his ‘punchline’ expression and says, “because it’s fucking close to water!” And then he waits for Sam to react.
Sometimes it’s all Sam can do to keep from blurting out you’re an idiot and I love you more than anything in the world. But that’s one of those things that’s REALLY better left unsaid, so instead he defaults to, “You really need to get some better material, man,” but he can’t ever quite keep the fondness from creeping through, which always makes the corners of Dean’s eyes crinkle up the way that means he’s counting the joke as a success.
“One of these days you’re going to laugh—I can tell you want to,” Dean claims, his expression one of deepest satisfaction.
“One of these days you’ll actually be funny and I’ll be too shocked to laugh,” Sam retorts, smile so big his face almost hurts.
Also new prompt-ficlet: Don't You Know, Everyone Wants to Laugh (for Sam and Jess and jokes)