Over his head

Me: "Knock-knock."
Christian: "Who's there?"
Me: "Eat mop."
Christian: "Eat mop who."
Me: "You eat your poo?
Christian: (cackles like he just heard the funniest joke in the world)(pauses) "I don't get it."
Me: "It's OK, I'm glad you don't get it."
Christian: "Did you make that up?"
Me: "No."
Christian: "Who did?"
Me: "I don't know."
Christian: "Well, I know my sister didn't make up that joke. And I know it wasn't God."
Me: "That's right. God only made one joke, and that was the platypus."
Christian: (more laughing) "I don't get it."


*** I should be relieved that he doesn't yet understand poop and fart jokes. However, I am worried that he doesn't understand what's so funny about the platypus. ***