A comedian in the family

About a month or so ago I was watching some show or movie with the girls and someone told a knock, knock joke and suddenly Camryn was hooked. Since then she has perfected one joke and has told it about 100 times (I don't really think that is an exaggeration). The joke goes like this:
Knock, knock.
Who's there?
A cow says.
A cow says who?
A cow says moo, not who!
It really was cute to watch her tell everyone she came in contact with the joke, but we thought it was time to move past this joke and so we started looking up more on the internet for her. Casey found one yesterday that he told to her and after he left for work she repeated it back to me, kinda.
Knock, knock.
Who's there?
Salad.
Salad who?
Lettuce in, it's cold out here! All I could do was laugh since I knew she meant to say lettuce where she had put in salad! We'll have to work on her delivery...