Knock Knock, Who is Present?

Whenever Mommy and Daddy approach another person's residence, they have the peculiar tradition of rapping their knuckles upon the door. The most surprising part, however, is that people actually respond by opening the portal and allowing us admittance. I have attempted to generalize this wherever I may in order to gain new friends from doorways, but as of yet, it appears mostly to be fruitless. Here I am, knocking upon our front doorway, hoping that somebody will open the portal and allow me to escape.