I've created a monster

I've started telling Joshua the "Knock-knock" jokes from my childhood. I know, I'm an idiot. It's like when I bought the boys plastic swords for their birthday and then was upset when they "stabbed" each other with them. However, what's worse than Joshua memorizing the jokes I tell him and telling them to me over and over is that he makes up his own. They go like this:

Josh: Knock-knock
me: Who's there?
Josh: Eyeball
me: Eyeball who?
Josh: Eyeball butthead!

And if you didn't find that funny, try this one.

Josh: Knock-knock
me: (sigh) Who's there?
Josh: Cup
me: (bracing myself) Cup who?
Josh: Cup, eyeballhead and your butt!

Bottom line, all of Joshua's knock-knock jokes have "eyeball" and "butt" in them and none of them are clever or funny. He's four.