Sheldon's Knock

Sheldon's knockSheldon exhibits a repetitive, ritualistic pattern when he's knocking on somebody's door.

Sheldon's knock consists of three knocks followed by calling out the person's name, which he performs three times for a total of nine knocks. Sheldon's knock is most associated with him going across the hall to Penny's apartment.

Sheldon is uncomfortable when he is interrupted mid-knock. Amy described Sheldon's knock as a sign of obsessive compulsive disorder, which Sheldon dismissed.

At the start of the series, Sheldon had an indistinctive knock. By the tenth episode, a characteristic knock was beginning to form. In that episode, "The Loobenfeld Decay", Sheldon performed a similar knock but with four knocks before Leonard's name, when he went to wake Leonard up in the middle of the night.

The first episode to feature Sheldon's knock as it would become was the second season episode "The Euclid Alternative", when Sheldon went to Penny's apartment to ask for a ride to work. Chronologically, the first time Sheldon used this knock was in a flashback scene in "The Staircase Implementation".

Featured in these episodes:

  • Season 1, Episode 10 - The Loobenfeld Decay

    Sheldon performs a ritualistic knock in this episode, but it's not quite the knock he would become known for. When Sheldon knocks on Leonard's bedroom door in the middle of the night, he knocks four times and says Leonard's name, repeating twice for a total of 12 knocks. Sheldon repeats this when he goes over to Penny's apartment. Sheldon's knock would eventually become 3 knocks & name x 3.

  • Season 2, Episode 5 - The Euclid Alternative

    Sheldon first uses his ritualistic knock (3 knocks & name x 3) in this episode when he goes to Penny's apartment to ask for a ride to work.

  • Season 2, Episode 6 - The Cooper-Nowitzki Theorem

    Sheldon uses his ritualistic knock when he rushes over to Penny's apartment after Ramona falls asleep, asking for Penny's help to get rid of her.

  • Season 2, Episode 7 - The Panty Pinata Polarization

    During Sheldon and Penny's feud, Penny uses Sheldon's knock when she angrily slams on his apartment door after he removed her laundry from the machines after she interrupted his laundry night.

  • Season 2, Episode 9 - The White Asparagus Triangulation

    Sheldon knocks on Leonard's bedroom in the middle of the night to show him Stephanie's Facebook page, where it says she is single and earlier threw a digital sheep at another guy.

  • Season 2, Episode 10 - The Vartabedian Conundrum

    After Stephanie tricks Sheldon into believing his larynx is inflamed, Sheldon uses a text-to-speech program on his laptop to say "Penny" when knocks on Penny's door.

  • Season 2, Episode 13 - The Friendship Algorithm

    Sheldon uses his knock when he goes to Penny's apartment to hand her a friendship questionnaire.

  • Season 2, Episode 18 - The Work Song Nanocluster

    Sheldon uses his knock when he takes a parcel for Penny over to her apartment.

  • Season 2, Episode 23 - The Monopolar Expedition

    Sheldon uses his knock twice in this episode. Once on Leonard's bedroom door during the middle of the night. When Sheldon knocks on Penny's door, she interjects "(knock) (knock) (knock) Sheldon" between each "(knock) (knock) (knock) Penny". Sheldon, unable to handle the change to his knocking ritual, keeps going until Penny eventually gives up and opens the door.

  • Season 3, Episode 1 - The Electric Can Opener Fluctuation

    Sheldon uses his knock twice when he angrily goes over to Penny's apartment to confront Leonard about the sabotage of his results on their North Pole expedition. The first time, Sheldon knocks for Leonard, then when he hears Leonard telling Penny to be quiet, he knocks for her.

  • Season 3, Episode 4 - The Pirate Solution

    Sheldon uses his knock when he goes to Raj's apartment to say Raj was right and to ask him to come back to working for him.

  • Season 3, Episode 6 - The Cornhusker Vortex

    When Sheldon interrupts Penny's football party to check on Leonard, he uses his ritualistic knock even though Penny's door was open.

  • Season 3, Episode 7 - The Guitarist Amplification

    Sheldon uses his knock when he goes over to Raj's apartment after Leonard and Penny's arguing upset him. Sheldon knocked on the door even though it was already open.

  • Season 3, Episode 8 - The Adhesive Duck Deficiency

    When Penny calls for Sheldon after she falls in the show, Sheldon uses his knock of every closed door in apartment. As he approaches the bathroom, Penny yells for him not to knock on the door.

  • Season 3, Episode 11 - The Maternal Congruence

    Sheldon uses his knock when he goes to check on Leonard in his bedroom. 

  • Season 3, Episode 13 - The Bozeman Reaction

    Sheldon uses his knock four times, each time for Leonard and Penny who are together.

  • Season 3, Episode 14 - The Einstein Approximation

    Sheldon uses his knock when he goes to wake up Leonard and Penny in Leonard's bedroom in the middle of the night.

  • Season 3, Episode 16 - The Excelsior Acquisition

    When Sheldon goes to Penny's apartment to prepare her as a witness in his case at traffic court, Sheldon starts his knock but Penny interrupts before the final round of knocking. After Penny performs the knock and says Penny, Sheldon says "That's just wrong."

  • Season 3, Episode 19 - The Wheaton Recurrence

    Sheldon knocks on Penny's door when he goes to give her ice cream, which he read in Cathy is an appropriate gift for stressed women.

  • Season 3, Episode 20 - The Spaghetti Catalyst

    Sheldon uses his knock on Penny's door when he finally gets to her apartment with his last remaining hot dog, after trading the others for his life.

  • Season 3, Episode 22 - The Staircase Implementation

    Sheldon knocks on Leonard's bedroom door when he interrupts Leonard with Joyce Kim. Historically speaking, this is the first time Sheldon performed his ritualistic knock.

  • Season 4, Episode 1 - The Robotic Manipulation

    Sheldon knocks on Penny's door when he goes to her apartment to ask for a ride to his date with Amy. When Penny tells him "You do realize I stand on the other side of the door waiting for you to finish knocking three times", Sheldon says he knows as he can see her feet under the door. After Penny says her point is it's a waste of time, Sheldon says an example of a waste of time is the conversation they're having.

  • Season 4, Episode 2 - The Cruciferous Vegetable Amplification

    Sheldon uses his knock three times in this episode, one for Leonard in the middle of the night and two for Penny. The final time Sheldon "knocks" on Penny's door, he uses his mobile virtual presence device (Shel-bot) to knock.

  • Season 4, Episode 4 - The Hot Troll Deviation

    When Howard goes to Penny's apartment to ask her for help patching things up with Bernadette, he knocks on the door like Sheldon and uses a voice recording of Sheldon saying "Penny!"

  • Season 4, Episode 6 - The Irish Pub Formulation

    After Leonard and Priya spend the night together, Sheldon uses his knock on Leonard's bedroom door when he hears a woman's voice in the room.

  • Season 4, Episode 11 - The Justice League Recombination

    When the guys go to Penny's apartment to apologize for their ridiculing of Zack, Leonard knocks on Penny's door three times and Sheldon feels compelled to say "Penny." Later, Sheldon, dressed as the Flash, knocks on Penny's door frantically, saying "Penny" each time.

  • Season 4, Episode 13 - The Love Car Displacement

    While the guys stay attend a science conference in Big Sur, Sheldon uses his knock of Raj's hotel room door after Leonard and Penny share a bed in the hotel room he was sharing with Sheldon.

  • Season 4, Episode 14 - The Thespian Catalyst

    Sheldon uses his knock twice, both on Penny's apartment door. The second time, as he goes to Penny's apartment for acting lessons, Sheldon frantically knocks and shouts her name. When Penny wonders what's wrong, Sheldon says he's acting.

  • Season 4, Episode 17 - The Toast Derivation

    When Sheldon grows tired of his alternative social group, he goes to Raj's apartment and knocks for "All my friends." As they were talking about Sheldon before he showed up, Howard quips "I think it's like Beetlejuice. We said his name too many times."

  • Season 4, Episode 18 - The Prestidigitation Approximation

    Sheldon uses his knock when he goes to Leonard's bedroom, while Leonard is with Priya, to perform a card trick. Leonard refuses to answer the door so Sheldon shows the cards underneath the door.

  • Season 4, Episode 21 - The Agreement Dissection

    Sheldon uses his knock twice on Penny's apartment. The first time to the use bathroom and the second time to complain about Priya.

  • Season 5, Episode 2 - The Infestation Hypothesis

    Sheldon uses his knock six times in this episode, five times on Penny's door and once on Amy's. Once in a daydream as he freaks out about Penny's chair. When Sheldon goes to her apartment to tell her to get rid of the chair, Penny answers the door to three of his knocks with pithy welcomes: "What's up, buttercup?", "What's word, hummingbird?" and "What's the gist, physicist?" After Shledon knocks on Amy's doors, she tells him his ritualistic knock is symptomatic of obsessive compulsive disorder, which Sheldon rejects.

  • Season 5, Episode 8 - The Isolation Permutation

    When Leonard takes Sheldon to Amy's apartment so he can talk to her, Leonard performs Sheldon's knock.

  • Season 5, Episode 9 - The Ornithophobia Diffusion

    When a bird won't get off the window ledge outside Sheldon and Leonard's apartment, Sheldon uses his knock on the window to tell the bird to go away.

  • Season 5, Episode 10 - The Flaming Spittoon Acquisition

    Sheldon uses his knock three times, twice for Penny, once for Amy and once for Penny, Amy and Bernadette. When Sheldon knocks on Penny's door, she interjects "Who do we love?" after each round of knocking before Sheldon says her name. When Stuart drops Amy off at her apartment after their date, Sheldon uses the knock from inside Amy's apartment to call for Amy. Finally, when Sheldon goes to Penny's apartment while Amy and Bernadette are there, he knocks on Penny's door and says one of their names for each round of knocking.

  • Season 5, Episode 16 - The Vacation Solution

    Sheldon uses his knock once when he goes to Amy's lab to work during his forced vacation.

  • Season 5, Episode 19 - The Weekend Vortex

    Sheldon uses his knock once on Penny's apartment door as he goes to give her a gift (Gummy Bears) to try get him out of spending the day at Amy's aunts birthday party as he promised.

  • Season 5, Episode 23 - The Launch Acceleration

    Sheldon uses his knock once when goes to Amy's apartment for date night.

  • Season 6, Episode 2 - The Decoupling Fluctuation

    Sheldon uses his knock twice, on Leonard's bedroom door and one over Penny's bed as she sleeps. After Sheldon heard that Penny was considering breaking up with Leonard, Sheldon was worried and wanted to tell his friend. Sheldon knocked on Leonard's door in the middle of the night to wake him up, only to say he still can't tell him. After that, Sheldon lets himself into Penny's apartment, goes to her bedroom, and knocks on the wall by her bed. Before Sheldon leaves, he asks Penny not to hurt his friend.

  • Season 6, Episode 6 - The Extract Obliteration

    As Sheldon desperately waits for Stephen Hawking to play another round on Words With Friends, Sheldon knocks on his iPad, calling for Stephen Hawking.

  • Season 6, Episode 7 - The Habitation Configuration

    When Sheldon goes to Amy's apartment to apologize for siding with Wil Wheaton, he knocks on her door, calling twice for Amy and finally for Angry Amy.

  • Season 6, Episode 10 - The Fish Guts Displacement

    Sheldon uses his knock twice on Amy's apartment door. The first time, Sheldon does two rounds of knocking before realizing he's calling her to go to a funeral, so he performs the last round solemnly. After leaving sick Amy in her apartment, Sheldon goes back and knocks again to tell her he made a commitment to comfort her in times like this. 

  • Season 6, Episode 12 - The Egg Salad Equivalency

    Sheldon performs his knock once on the door of Janine, the head of human resources at the university, calling for "Human Resources Department" as he knocks.

  • Season 6, Episode 15 - The Spoiler Alert Segmentation

    When Sheldon gets scared that Amy is planning to move into his apartment, he knocks on Penny's door, calling for his "Good buddy Leonard", in a bid to get Leonard to return home.

  • Season 6, Episode 17 - The Monster Isolation

    When Sheldon goes to thank Penny for giving him acting tips, he gets to her apartment as she's just opening the door. Penny asks if Sheldon will okay since he didn't get to perform his knock, which he says he will since he didn't start. Nevertheless, Sheldon performs the knock as he talks to Penny, inserting a round of knocking each time he's about to say her name.

  • Season 6, Episode 20 - The Tenure Turbulence

    Sheldon uses his knock twice on the office of Mrs. Davis, the Human Resources director.

  • Season 6, Episode 23 - The Love Spell Potential

    After Amy gets upset that the guys are mocking her relationship with Sheldon during their game of Dungeons and Dragons, she runs to his room to be alone. When Sheldon goes to check on her, he performs his knock on his own bedroom door, calling for Amy. Sheldon says "I've never knocked on my own door before. That was a wild ride."

  • Season 7, Episode 1 - The Hofstadter Insufficiency

    When Sheldon has a nightmare while Leonard's away, he runs over to Penny's apartment and knocks on her door in the middle of the night, asking if he can sleep over.

  • Season 7, Episode 1 - The Hofstadter Insufficiency

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  • Season 7, Episode 2 - The Deception Verification

    After Sheldon and Amy listen from outside Penny's apartment to a male voice, Sheldon barges in to see whether it's really Leonard. When Penny tells Sheldon he can't just barge in, he goes back outside and performs his k

  • Season 7, Episode 5 - The Workplace Proximity

    When Sheldon goes ever to Penny's apartment to talk to Amy during their girls' night, he knocks on the door and calls for "Amy, Bernadette, Penny." After Bernadette wonders whether Sheldon is ever going to stop his ritualistic knocking, Amy says she doesn't mind as she's hoping to put his love of repitition to good use some day.

  • Season 7, Episode 7 - The Proton Displacement

    Sheldon uses his knock once on Professor Proton/Arthur's door. Sheldon remarks on Professor Proton's door knocker, saying "That's TV money."

  • Season 7, Episode 11 - The Cooper Extraction

    When Penny asks Leonard if he ever imagines what it would be like to have not been born, Leonard mocks Sheldon's knock, adding "What do you think?" between the rounds.

  • Season 7, Episode 12 - The Hesitation Ramification

    Amy tells Bernadette it's exhausting that Sheldon is trying to use science to determine the basis of humor. Amy asks her if she has any idea how hard it is to laugh at a knock-knock joke that starts with "Knock-knock-knock, Amy. Knock-knock-knock, Amy. Knock-knock-knock, Amy?"

  • Season 7, Episode 12 - The Hesitation Ramification

    After Sheldon and Leonard talk in the hallway, Leonard goes and knocks three times on Penny's door. Sheldon, who is opening his own apartment door, feels compelled to say "Penny."

  • Season 7, Episode 16 - The Table Polarization

    Sheldon uses his knock once on Amy's door when he goes to break up with her.

  • Season 7, Episode 18 - The Mommy Observation

    Sheldon uses his knock once on the door of his mother's house in Texas.

  • Season 7, Episode 20 - The Relationship Diremption

    When Amy leads a drunk Sheldon back to his bedroom, Sheldon knocks on his own door, saying "Empty room" between rounds, before they enter. Sheldon remarks that "if somebody says come in, I'm gonna freak out."

  • Season 7, Episode 21 - The Anything Can Happen Recurrence

    When Sheldon, Leonard and Penny see Amy and Bernadette in a bar when they said they were busy, they wonder why their friends lied to them. To find out, Sheldon knocks on the bar window, calling for Amy and Bernadette, before asking "Why did you lie to us?"

  • Season 8, Episode 2 - The Junior Professor Solution

    Sheldon uses his knock once when he goes to Mrs. Davis' office. After the first round of knocking he says "Mrs. Davis", after the second he says "Guess who?", after which she answers. Finally, Sheldon says "That's right, good job." after the third round of knocking.

  • Season 8, Episode 5 - The Focus Attenuation

    When Sheldon goes to Penny's apartment to talk to her about the party the girls are throwing, he does his ritualistic knock but with five rounds. After Penny questions why he knocked more than usual, Sheldon says "Next time I might be in a rush. It's good to have a few in the bank."

  • Season 8, Episode 10 - The Champagne Reflection

    Sheldon performs his knock once at LeVar Burton's house. 

  • Season 8, Episode 13 - The Anxiety Optimization

    Sheldon uses his knock once when he hears noises coming from Penny's apartment.

  • Season 8, Episode 14 - The Troll Manifestation

    Sheldon uses his knock once on Penny's apartment door when he goes to tell Leonard he spoke to the magazine reporter about the coverage of the paper they co-wrote together.

  • Season 8, Episode 21 - The Communication Deterioration

    When Penny goes over to Sheldon's apartment to ask for his advice, she uses his knock. After Sheldon says "I bet that started off as a joke, but by the third one you realized there was something strangely enjoyable about it.", Penny says she kind of wants to do it again.

  • Season 9, Episode 1 - The Matrimonial Momentum

    When Sheldon went to Penny's apartment with a hot beverage to comfort her, he was unable to knock on his door as his hands were busy holding a tray. Instead, Sheldon said his knock out loud.

  • Season 9, Episode 2 - The Separation Oscillation

    When Sheldon goes to Amy's apartment to give her a box of her belongings following their separation, Sheldon doesn't perform his characteristic knock. Amy believes it's because he wanted to make sure she opened the door to him, but Sheldon tells her it's because she no longer gets to enjoy his "charming eccentricities".

    After Amy goes back into her apartment, Sheldon performs the rest of his ritualistic knock on a table in the corridor, remarking "Tables work, too. Good to know."

  • Season 9, Episode 10 - The Earworm Reverberation

    Sheldon performs his knock on Amy's apartment door when he rushes over there to try win her back. Despite his date with Amy being interrupted by her ex-boyfriend, Dave is more excited to finally meet the great Sheldon Cooper.

    "Amy: That's Sheldon.

    Dave: You're kidding. How's my hair?"

  • Season 9, Episode 11 - The Opening Night Excitation

    When Sheldon goes over to Penny's apartment to see her and Bernadette, they decide to have some fun with Sheldon's door knocking routine. When Sheldon knocks on Penny's apartment door, Bernadette tells Sheldon to come in, instead of Penny. This causes Sheldon to repeat his habitualistic known but calling for Bernadette. When Penny answers this time, Sheldon tells them to "Keep it up. I got nowhere else to be", at which point Bernadette tells Sheldon to just come in. Greeting the ladies, Sheldon tells them "For future reference, if I want to watch Mean Girls, I’ll stream it on Netflix."

  • Season 9, Episode 14 - The Meemaw Materialization

    Sheldon knocks on Penny's apartment door after being sent away from Amy and his Meemaw's chilly conversation in his own apartment.

  • Season 9, Episode 21 - The Viewing Party Combustion

    Sheldon knocks on Leonard's bedroom door in the middle of the night to ask him an inane question about Batman that he'd not been able to ask earlier.

  • Season 9, Episode 23 - The Line Substitution Solution

    Sheldon performs his ritualistic knock on Penny's apartment door when he goes to apologize to Amy for sending Stuart to spend time with her while he waited in line at the movies. Earlier, Bernadette wondered whether Sheldon had come to apologize to Amy when there was a knock at the door, but it turned out to be Stuart sent by Sheldon to apologize. Bernadette didn't seem to notice that Stuart's knock on the door was not Sheldon's characteristic knock.