Why Sheldon Cooper Always Knocks Three Times On The Big Bang Theory

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Everyone who's ever watched "The Big Bang Theory" knows that every time Sheldon Cooper (Jim Parsons) enters a dwelling, he has to knock on the door exactly three times, sort of like a very particular vampire. But why does he do this, and how did the writers come up with it?

In Jessica Radloff's 2022 book "The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series," some of the writers and performers who worked on "The Big Bang Theory" explained the origin of Sheldon's three-knock system, which boils down to this: it started as an innocuous thought by writer and producer Lee Aronsohn, who just thought it was funny, but in season 10 of the show, Sheldon tells Penny (Kaley Cuoco) the real reason behind the triple knock. As Sheldon reveals, he walked in on his father with a woman who was not his mother when he was 13, and now he's afraid to knock only once. "The first is traditional, but two and three are for people to get their pants on," Sheldon explains to Penny.

"We weren't looking for an opportunity to justify that behavior, but it came up really organically in the room, where he's talking to Penny and he says this is why I knock three times," writer and co-executive producer Tara Hernandez told Radloff. She continued:

"It scared a lot of people because something that was played off as really silly and a joke was actually rooted in something that was kind of traumatic for Sheldon. Moments like that, that are kind of scary, are always my favorite in a writers room because people are going to react very strongly. We talked about it a lot, but ultimately we trusted that it felt real for Sheldon so it should go in the show, whether or not that brought up complicated feelings for people or not."

Kaley Cuoco also reveals in the book that she had her own interpretation of what the "knocks" meant, and it had to do with Penny's elusive, never-revealed maiden name (the character doesn't get a last name until she marries Johnny Galecki's Leonard Hofstadter), and it was all thanks to one fan's comment. "I think it was a fan who once said to me, 'Wouldn't it be funny if your name was Penny Penny Penny, and Sheldon knew the whole time and then everyone just started doing the knock and then later he was like, 'I knew that was your last name!'" Cute idea, but the show went with a slightly darker interpretation.

As Lee Aronsohn told Jessica Radloff in the book, Sheldon's three-knock system was just a funny little bit he came up with ... and he never knew it would take on a life of its own. "It was just one of those things where the stage direction was Sheldon knocks on the door, Penny answers. And I just said, 'What if he does this?'" Aronsohn recalled. "And I demonstrated that instead of one knock and Penny opens the door, it was three knocks. It showed Sheldon's obsessive-compulsiveness." Aronsohn explained, "People like that like patterns and structure, so we had some fun with Sheldon once that got established. There was one time he knocked on her door twice, and on the second one she opened the door, and Sheldon could just not continue! He struggled and struggled and struggled and finally had to go back to the door and go, knock, knock, knock, Penny."

Peter Chakos, an editor and co-producer on "The Big Bang Theory," found it amusing in the moment, but like Aronsohn, he had no idea that it would become, for lack of a better term, a whole thing. "I was there when Lee did it," Chakos said. "It was just a moment where Sheldon knocked once and Penny answered. I saw Lee acting it out in his head and thinking, If Sheldon knocks three times, is that funny? And then he pitched it, and that became fodder for the series."

Aronsohn left the show as a writer in 2011 after the fourth season, but here's the good news: he approves of Tara Hernandez's clever retcon of the three knocks. "I was gone by the time they came up with that, but that's brilliant reverse engineering," Aronsohn told Radloff. "Certainly, from a story and character point of view, I love the explanation that he obviously was traumatized by walking in on his father [...] And I was gone by that time. So kudos."

Steve Holland, a longtime writer on "The Big Bang Theory" who eventually became an executive producer, recalled — as Tara Hernandez did — that some people weren't wild about the thought process behind Sheldon's three knocks. "We came up with that reveal kind of in the moment, but I remember some crew people being upset by that revelation," Holland remembered. "One person said, 'It's upsetting that that is why he does that. I don't like that his knock is from this darker place.'"

Ultimately, the three-knock thing caused a tiny continuity problem for the first-ever spin-off of "The Big Bang Theory," the single-camera prequel "Young Sheldon," which actually provides a different explanation for Sheldon's system. As Steve Molaro — another writer and executive producer who ended up co-creating "Young Sheldon" alongside Chuck Lorre — told Radloff, "I think the real answer is, we were just trying to find a way to add some weight and importance to the scene when he said it, and that was a way to do it. Had I known it would end up having an impact on 'Young Sheldon,' I might have thought twice about it." On "Young Sheldon," a season 7 episode shows a 15-year-old Sheldon walking in on two adults engaging in what Sheldon would call "coitus," his father with a "strange woman" ... except that woman is his mother Mary (Zoe Perry) dressed as an Oktoberfest maiden after she and Sheldon spent the summer in Germany.

The three-knock system also caused a little confusion later on in "The Big Bang Theory" after Sheldon moves out of his and Leonard's apartment to live with his wife Amy Farrah Fowler (Mayim Bialik). "And as writers, we discussed what the rules would be for Sheldon going forward," Holland mused. "Does he have to do his 'knock, knock, knock' when he comes back into his apartment? But maybe if we leave the door cracked to his old apartment, he doesn't have to knock!"

You can see all of Sheldon's special knocks on "The Big Bang Theory," which is streaming on Max.