KNOCK KNOCK (4 YRS OLD)

After momma picked up S from preschool, S told momma a new knock knock joke.
S- knock knock
M - who's there?
S - banana
M- banana who?

S - knock knock
M - who's there?
S - banana
M - banana who?

S- knock knock
M - who's there?
S - banana
M- banana who?
S- orange you glad I didn't say banana

This is the first time momma has ever heard this knock knock joke from her. Where did she learn it?

P.S. When momma told daddy this story, he said that the last knock knock line should have been "orange." Momma guesses that S has momma's memories for jokes and punchline.