The act of bored children knocking on a door, and reciting the alphabet, which ever letter they get to when the occupant opens the door, is the first letter of the Person they MUST ask for.
No real purpose but to impose serious embaressment on the knockee.
Kid 1: Who wants to play Knock-a-Door ABC?
Kid 2: ME!
*walks to a random door*
Kid 2: *Knock Knock* A B C D E F G H I J K *door opens*
Kid 2: Hi is keith there please?
Owner: There is no keith that lives here.