"OK, everyone, take your places. I've done the math, and this one should be 5.8 +/- 0.4 Couliers worth of funny."
The theories aren't completely worthless, however, and one condition that shows up in many of them is that for a joke to work, it needs to be surprising. The audience can't see the joke coming. If that seems obvious to you, consider that it applies to more than just jokes repeated verbatim; it applies to formats and structures as well. Which is why, if you've watched sitcoms your whole life, you can turn to ... well, nearly everything on CBS, and see all the jokes lumbering slowly toward you like sad, hilariously doomed dinosaurs. The jokes are still original, and funny in their own way, but because they follow the same structure and rhythm sitcoms have used for the last 50 years, they're a lot less enjoyable to watch. A human being can tolerate only so many misunderstandings based on the different definitions of the word "poke" before they snap.
How to Do Better
The lesson is for you to try to tell jokes that people haven't heard before.
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"OK, in this joke, Priest, you and the Farmer's Daughter are going to quietly study the Bible, respecting her father's reasonable demands for putting you up for the night. Banana Peel, I'm going to need you to watch from the shadows and beat off. Aaaaand ... ACTION."