Sheldon Lee Cooper, Ph.D., Sc.D., is a fictional character in the CBS television series The Big Bang Theory, portrayed by actor Jim Parsons. He is often described as the stereotypical "geek". He is usually characterized as extremely intelligent, socially inept, and rigidly logical. Despite his intellect, he regularly displays a lack of common sense. He has a superiority complex, but also possesses childlike qualities, of which he seems to be quite unaware.
We all assumed that since Sheldon Cooper is an exceptional person, hence he has eccentric habits. One of the thousands of them is knocking on a door at least three times. But boy, were we wrong to assume that! Turns out, there's a whole story behind his ritual of knocking three times.
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The real reason behind his three knocks was revealed on a recent episode of The Big Bang Theory (Season 10, Episode 5 - The Hot Tub Contamination) that was aired on 17th October, 2016. He opens up to Penny when she asks him, "C'mon! What is really going on with you?"
This is how their conversation goes down-
Penny: C'mon! What is really going on with you?
Sheldon: Penny, I am going to tell you a story that I've never told anyone.
Sheldon: I was 13 years old, and, on spring break from college.
Penny: Not relating. Go on.
Sheldon: I came home early because they ran out of Math to teach me.
Penny: Oh, now I'm with ya! Right.
Sheldon: My mother was in bible study. I walk in the house expecting to find it empty and I heard a sound, coming from my parents' bedroom. When I opened the door, I saw my father having relations with another woman.
Penny: Ugh, that's awful.
Sheldon: I know. That's also because I don't open a door without knocking three times. I mean, the first one is traditional, but two and three are for people to get their pants on.
Reading is fun too, but it's always more fun to watch Sheldon Cooper. See, the video below, it covers the above conversation and more.
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So, there it is! The real reason behind the ritual of those three knocks on a door by Sheldon Cooper has finally been revealed.