The Music of a Door

Who was it, do you think, who first decided to tap out a certain rhythm on a door? Knock-kn-knock-knock, knock-knock.

How long ago was that? And why?

We all have a way to knock on a door, some of us barely tap, others pound forcefully. Our own tunes are different, slight differences in how many times we touch the door itself and in what order or rhythm.