Rory's First Knock Knock Joke

Ok, well, first one that made sense anyway!
Knock knock.
Who's there?
Cargo.
Cargo who?
Cargo beep beep!

It's so cute, he's trying SO HARD to be a comedian, but he's just not that good at it yet. So he's been trying all these ridiculous knock knock jokes but can never remember the punch lines, and then telling the chicken crossing the road joke to death, and his new thing is just to repeat the last word he said and then say, "get it?" Like, "I see a blue car. Car, get it?" and then he laughs and laughs...and of course, I laugh too, which just encourages him!