‘The Big Bang Theory’: When Did Sheldon Develop His Need to Knock Three Times Before Entering a Room?

For 12 seasons, fans followed the lives of Sheldon Cooper, Leonard Hofstadter, Raj Koothrappali and Howard Wolowitz on The Big Bang Theory. During those 12 seasons, it became pretty obvious that the four geeky pals were each incredibly complex and deeply flawed, but none were near as eccentric as Sheldon. Sheldon had a ton of interesting quirks, and his knocking habits were among the most discussed by fans. Do you know how he developed his odd knocking sequence?

What was Sheldon’s signature knock?

Sheldon’s most well-known peculiarity was the way he knocked on a door to make his presence known. Instead of knocking once and waiting for a reply, Sheldon would knock on a door three times, state the person’s name, knock three more times, state the person’s name, and repeat the sequence a third time before he would enter a space. The peculiar knocking sequence may have started as just another one of Sheldon’s quirks, but it actually had a pretty deep and dark origin story.

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Sheldon, while a genius, proved to be one of the most complex and deeply flawed characters in the series. While he was honest about how his upbringing in Galveston, Texas, was difficult, he didn’t reveal just how imperfect his family was right away. In fact, he waited several years to tell his friends about some of the more traumatic episodes in his life, and the reason behind his strange knock was one of the biggest reveals of the entire series.

Where did the knock come from?

Sheldon’s knock became a part of the series early on, but an explanation for the obsessive knocking sequence eluded fans until season 10. In the episode titled, “The Hot Tub Contamination,” Sheldon confides in Penny that he knocks in three series of three because of childhood trauma. He explained that he caught his father cheating on his mother because he failed to knock long enough for his father and his mistress to get their clothes on.

Sheldon Cooper and Mary Cooper
Sheldon Cooper with his mother, Mary Cooper appear on ‘The BIg Bang Theory’ | Sonja Flemming/CBS via Getty Images

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Sheldon went on to explain that the first three knocks are tradition, while the additional knocks are intended to give people enough time to get their pants on. He didn’t say so, but it seems likely that his penchant for stating the person’s name is also tied to his childhood trauma. Sheldon admitted that he never told anyone about his father’s affair, and, presumably, Mary Cooper had no idea what was going on.

Was Sheldon’s signature knock a plothole?

Sheldon explained that he developed the compulsion to knock multiple times after accidentally walking in on his father cheating on his mother. According to Sheldon’s statement, the event took place when he was 13-years-old, and he had knocked the same way ever since, but that’s not actually the case. In the second episode of The Big Bang Theory, Sheldon knocks on a door with a single knock, which is traditional. According to Fandom, his signature knock was not featured until the fifth episode of season 2. In the episode, Sheldon knocked on Penny’s door, in the sequence that would become his signature, to get a ride to work.

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After season 2, Sheldon knocked in his signature sequence every single time he knocked on a door under typical circumstances. Throughout season 1 and the first few episodes of season 2, Sheldon employed a different knocking sequence, which suggests, perhaps, his story about his father was not true. Was it a true plothole? Fans are divided on that. It is absolutely a smaller plothole than, say, Sheldons’ allergy to cats, or the timeline of the broken elevator.