knock knock

When my grandparents were young:
(From what I've heard) People didn't lock their front doors. Neighbors might even walk right in without knocking. Actually, apparently some people still are like that.

When I was young:
People locked their doors. If someone knocked on it or rang the doorbell, you'd open it (after looking through the peep-hole to see who it was). "Knock, knock. Who's there?" was a popular category of jokes.

Nowadays:
Someone knocks on a door. The person inside doesn't answer it, but posts to Nextdoor warning all their neighbors to watch out for a person who's going around knocking on doors, along with a detailed description of the person.