Progression of Joke Telling

Peyton made up a new joke the other day. And it got me thinking about how my little boy's humor has changed as he's grown.
Remember the "Diarrhea" song in "Parenthood" as the family is driving home?
"As you're sliding into first and you feel a little burst - Diarrhea! Diarrhea!" and the kids are absolutely cracking up as they sing it. Remember that? Yep. My son is at that age.

Here was his best joke when he was 3 years old:
He would stop whatever he was doing, walk over to me and yell-whisper, "PEANUT BUTTER!"
And then he would crack up. So innocent and sweet.

Then he turned 4 years old and started to get the basic idea of what a joke really was:
Peyton: "What's a tree and an elephant like?"
Me: "What?"
Peyton: "A TRUNK!"
And then he would crack up. Sure, he was telling the joke the wrong way, but he understood the basic principle.

And since he understood the basic principle of joke telling, he was able to put his own spin on it:
Peyton: "Mommy, what's an iPhone?"
Me: "I don't know, what is it?"
Peyton: "A phone with POOP on it!"
And then he would crack up. Here is when I realized that EVERYTHING is funny with poop on it.

And this is the new joke he told the other day, he's 5 years old:
Peyton: "Daddy, can you marry a pig?"
James: "No."
Peyton: "Because they don't have VAGINAS!"
Oh My Dear Lord, you have never heard a kid laugh so hard. And I know that this joke is just weird on so many levels, so I try not to think about it. But this is when I realized that EVERYTHING is funny with private parts attached.

Awesome. What could possibly come next in the land of boy jokes?