Repeat a joke once or twice and it's still funny, but repeat it too much and it just gets annoying. Keep on going, though, and that repetition can make the gag hilarious again, getting a sitcom audience to howl with laughter and feel like they're being greeted by an old friend.
That's exactly why running jokes work so well on television, those little moments and lines that pop up all throughout a series, and the most memorable tend to be the ones that last a ridiculously long time. They're the gags writers stumble upon and then repeat and expand, establishing a sense of continuity to the series. After all, television comedy tends to be pretty episodic, and so this is an effective way to connect the current episode to the show's storied past.
The longer the joke goes, the funnier it often is, just because you can't believe how many damn years the writers have kept it up. Hell, with some of these examples, a kid could be born during the first instance of the gag, and he'd be driving by the last instance.
From slaps to libraries to tripping over steps, here are 10 great running gags that lasted an insanely long time.