These are the classics. Dedicated to LynnJems who helped with part of this.
Me: Knock Knock.
Friend: Oh no.
Me: KNOCK KNOCK!
Friend: *sighs* who is there?
Me: Doctor.
Friend: Doctor Who.
Me: *in British accent* Just the doctor, well the tenth.
HAAHHAHAHAHAHAHAH!
Friend: hmmmmmmmm
Me: Knock knock.
Friend: Who is there?
Me: You know.
Friend: No I don't.
Me: You ruined it. Your supposed to end with who.
Friend: You-know-who.
Me: Voldemort. Hehehhehhhehehehehehhe.
