Here is another knock knock joke, one which follows the standard pattern, but is more fun because it involves cows:

Informant: Knock knock

Me: Who’s there?

Informant: Interrupting cow

Me: Inter[MOOOOOO!]

[a pause]

Informant: You’re supposed to go “interrupting cow who?” and then I… interrupt them.

While the informant did not recall exactly where he heard this joke, he did remember to have heard it or its variations multiple times while working with young children. It’s silly formula and absurdity surely appeal well to children, and it seems that it likely arose first from among children themselves