knock knock What’s the matter with the crying inside?

nock, knock jokes refer to using homophonic words to make a sentence. Give an example and you will understand:

(knock, knock)

Who is it?

（Will.）

（Will） who?

（Will you please let me in)?

HA- HA!

{The part in brackets is the main content of the joke and can be replaced. }

It's like the trick in a Chinese sketch is "Dang Dang Dang" and then the performance starts. It's not a real sound