My daughter doesn't really get knock knock jokes, but she likes to do the interrupting cow one over and over like I've never heard it from her before. So I said to her:
Me: Knock Knock
Leah: Who's there?
Me: Interrupting Zombie
Me: Uuuuuuuuuuuuurrrrrggghhhhh BRAINSSSSsss!!!!
Then I try to "eat her brains". She doesn't get knock knock jokes, but she knows how to take down a zombie.