Someone who shows up to your door for no reason.
Czech before you open the door !
Bolzano the door !
A:.........
Well hell-if-i-know!
There's none. They both knock on the door, but never goes in!
Nobody asks, 'who's there ' when you try and tell a knock knock joke.
Because they don't believe in higher powers.
DogMa
Just another reason to teach your cat to read.
Dads joke) The same reason 10 pennies is worth more than 9.