A Jehovah's Witness.
An extrovert mathematician will be looking at the other guy's shoes.
Three. When you tell it to her, then when you explain it to her, and then when she gets it.
There's none. They both knock on the door, but never goes in!
Someone who knocks on your door at 6 a.m. for no reason.
knock knock knock... excuse me sir, but do you have a few minutes to discuss nothing
I am the one who Knock-knocks.
Because Freedom Rings!