Sheldon's Knock

Sheldon has a particular  when alerting someone to his presence outside their door. Due to his tendencies, he must finish his knock, even if the person he's alerting opens the door before he is finished, in which case he will usually finish it on the door frame.

His knock consists of three knocks before stating the person's name, and then he repeats this process two additional times:


 * (knock, knock, knock) "Penny?"
 * (knock, knock, knock) "Penny?"
 * (knock, knock, knock) "Penny?"

This knock is mostly associated with his trips to Penny's apartment across the hallway, but is his general knock for any character in the series. Sheldon will also use this knock on the wall above someone's head if he enters their bedroom and is trying to wake them up.

Origins and development
The first time we see Sheldon knock on someone's door was in the second episode of the first season called "The Big Bran Hypothesis", and due to the infancy that the series was in, Sheldon's knock was nothing unusual. It wasn't until episode ten that his knock had become unique. Sheldon's signature knock has gone through several changes in the series before his signature knock was created.

The earliest version of his signature knock was shown in "The Loobenfeld Decay". This featured three or four knocks before saying the person's name, and it continued until they answered. In this episode, it was on Leonard's and Penny's doors. His current knock (three knocks and the name; three repetitions) officially caught on in "The Euclid Alternative" and became his signature knock with few exceptions in which he varied it throughout the series.

In-universe, Sheldon's earliest time to have used this knock was in the flashback episode "The Staircase Implementation", which takes place before the start of the series. It's unclear if his knock was actually different at this time, because the only scene it's featured in depicts it slightly different, being slower. This difference could have been due to the situation of in which he used it.

In "The Habitation Configuration", Sheldon is drunk and knocks on Wil Wheaton's door two times instead of three.

Sheldon's knock used by others
Throughout the series, Sheldon's knock has been ed by other characters as either a or to make the person behind the door think it is Sheldon doing the knocking:


 * "The Panty Piñata Polarization", Penny angrily mimics the knock when she confronts Sheldon over her missing laundry
 * "The Vartabedian Conundrum", when Sheldon thinks he lost his voice, he uses a computer-generated voice in place of his own to say "Penny"
 * "The Hot Troll Deviation", Howard uses a recording of Sheldon saying Penny's name to copy his knock so Penny would think it was Sheldon, because he knew she wouldn't answer the door if she knew it was him and wanted an update on Bernadette
 * "The Isolation Permutation", Leonard knocks on Amy's door like Sheldon in order to make Sheldon comfort her when she was upset
 * " The Monopolar Expedition", Sheldon is chosen to go to the magnetic North Pole to try and confirm  string theory. Leonard, Howard and Raj are allowed to join him. Sheldon uses this knock twice for Penny. The second time, she stands on the other side of the door and repeats his knock to him, saying his name. She finally opens the door after several times.Dwg4.jpg

Episodes

 * "The Loobenfeld Decay" (S1E10): Penny is singing in a production of Rent, and Leonard and Sheldon don't want to have to hear Penny's singing. Sheldon develops an elaborate ruse into why him and Leonard can't go to Penny's performance. Sheldon uses the earlier version of his knock four times in the episode. Three times with Leonard and once with Penny.
 * "The Euclid Alternative" (S2E5): Leonard works the night shift, so Sheldon can't drive himself to work. Sheldon uses his knock once with Penny. This is the first time that he uses the knock that he is known for later on.
 * "The Cooper-Nowitzki Theorem" (S2E6): Sheldon has a woman at the University who is interested in him and in working with him. Sheldon uses this knock once with Penny, and she makes mention that he doesn't need to knock so much as she can hear him due to her small apartment.
 * "The Panty Piñata Polarization" (S2E7): Penny is given a third strike and thus banished from the apartment by Sheldon; Penny and Sheldon fight. Penny uses it once to ask him where he put her clothes that were in the dryers in the laundry room. Sheldon uses it once for Penny to concede (Leonard had gotten Penny to call Sheldon's mother and tattle on Sheldon to end the fight).
 * "The White Asparagus Triangulation" (S2E9): Sheldon tries to set up Leonard with Dr. Stephanie Barnett. He uses the knock three times; once with Leonard (to show Leonard that Stephanie Barnett's relationship status on her web profile was still 'single') and twice with Penny (to get advice from Penny on how to get Leonard and Stephanie together).
 * "The Vartabedian Conundrum" (S2E11): Dr. Stephanie Barnett performs the "Sheldonectomy" - tricks Sheldon into thinking that his larynx is inflamed and must stop talking immediately. Sheldon uses his knock once with Penny, but uses a voice program on his laptop to say her name.
 * "The Friendship Algorithm" (S2E13): Sheldon tries to scientifically discover the secret of friendship, so that he can become friends with Kripke. He uses his knock once with Penny, handing her a multi-page friendship questionnaire.
 * "The Work Song Nanocluster" (S2E18): Penny tries to become self-employed, making Penny Blossoms with Sheldon's business knowledge assistance. Sheldon uses this knock once for Penny. He is delivering a package that was left with him when she wasn't home.
 * "The Monopolar Expedition" (S2E23): Sheldon is chosen to go to the magnetic North Pole to try and confirm string theory. Leonard, Howard and Raj are allowed to join him. Sheldon uses this knock twice for Penny. The second time, she stands on the other side of the door and repeats his knock to him, saying his name. She finally opens the door after several times.
 * "The Electric Can Opener Fluctuation" (S3E1): Sheldon's work trip to the North Pole was sabotaged by Leonard, Raj, and Howard since Sheldon was becoming unbearable and the only way that to make Sheldon bearable was by producing false scientific results for Sheldon; Leonard and Penny become a couple upon Leonard's return from the North Pole. Sheldon uses this knock once for Leonard (while Leonard and Penny are kissing). When Leonard tells Penny to not make a sound, Sheldon then uses the knock once for Penny.
 * "The Pirate Solution" (S3E4): The university has cut the funding to Raj's department, and he is going to be deported if he looses his job. He gets a job working for Sheldon. Sheldon uses this knock for Raj.
 * "The Cornhusker Vortex" (S3E6): Penny has her friends over to watch a football game, and Leonard goes to watch the football game in an attempt to fit in with Penny's friends. Sheldon uses this knock once during Penny's party, even though her door is open.
 * "The Guitarist Amplification" (S3E7): Penny and Leonard's arguing upsets Sheldon. Sheldon uses this knock once for Raj. Raj yells at Sheldon to come in since the door is open, and then requests that Sheldon tell Raj's parents that their research is at a critical juncture and that Raj can't go home to India to attend his cousin Sanjay's wedding.
 * "The Adhesive Duck Deficiency" (S3E8): Penny slips in the shower and dislocates her shoulder; Sheldon has to drive Penny to the hospital for medical care. He uses this knock on all the closed doors in Penny's apartment on his way back to her bathroom. She tells him to not knock when he gets to the bathroom door.
 * "The Maternal Congruence" (S3E11): Leonard's mother comes for a visit and divorces from Leonard's father. Sheldon uses this knock once for Leonard.
 * "The Bozeman Reaction" (S3E13): Sheldon and Leonard's apartment is burglarized. Sheldon uses this knock four times, all for Leonard and Penny who are together.
 * "The Einstein Approximation" (S3E14): Sheldon spends days trying to figure out how electrons move through graphene as if they have no mass. He uses this knock once after entering Leonard's bedroom to wake Leonard and Penny up.
 * "The Excelsior Acquisition" (S3E16): Sheldon gets a court summons from the time he drove Penny to the hospital, Sheldon is determined to fight the charges and Penny will accompany him to court. He uses this knock once for Penny - however before Sheldon can do the third triple knock, Penny opens her door, does the triple knock and says "Penny?", to which Sheldon replies, "Well, that's just wrong."
 * "The Wheaton Recurrence" (S3E19): A bowling bet between Sheldon and Stuart. Stuart has Wil Wheaton on his team, and Wil Wheaton breaks Penny and Leonard up in order to make Sheldon's bowling team lose. Sheldon uses his knock once to give Penny ice cream (which according to studying the comic strip Cathy, is appropriate to give to a woman under stress).
 * "The Spaghetti Catalyst" (S3E20): Leonard and Penny are broken up, Sheldon is divided into whether he should spend time with Leonard and the guys or whether he should spend time with Penny. Sheldon uses this knock once for Penny (Sheldon has only one hot dog left, having had to give up the other five to a mean angry dog in order to save his life).
 * "The Staircase Implementation" (S3E22): Flashback episode. Leonard and Sheldon are arguing over the thermostat, and Sheldon goes to Penny's apartment and recounts to Penny how he and Sheldon became roommates. Sheldon uses this knock once for Leonard and Joyce Kim.
 * "The Robotic Manipulation" (S4E1): Sheldon thinks about having offspring with Amy Farrah Fowler. Sheldon uses this knock once for Penny, after which she explains that she stands on the other side of the door, waiting for him to finish, implying that it's a waste of time. Sheldon states that he was aware of that, being able to see the shadow of her feet under the door.
 * "The Cruciferous Vegetable Amplification" (S4E2): Sheldon is worried about getting hurt and wants to exist as a robot. He uses this knock three times, once for Leonard (when he has pain in his abdomen from eating so many vegetables) and twice for Penny (once to go jogging with Penny, and the second time, using his robot in order to ask Penny to sing "Soft Kitty" to him).
 * "The Hot Troll Deviation" (S4E4): Using his avatar in his World of Warcraft game, Howard cheats on Bernadette. Howard them uses a recording of Sheldon's voice and his signature knock to get Penny to come to the door.
 * "The Irish Pub Formulation" (S4E6): Leonard and Priya are in a relationship, trying to keep the relationship secret. Sheldon uses his knock once for Leonard (while Priya is in Leonard's bedroom, Leonard tries to keep Priya's presence concealed from Sheldon).
 * "The Justice League Recombination" (S4E11): The guys befriend Penny's boyfriend Zack and get him to dress up as Superman for the comic book store New Year's Eve costume contest. The guys want Penny to come to the costume contest dressed as Wonder Woman. Sheldon uses his knock for Penny as the Flash, which consists of him knocking and saying "Penny" repeatedly.
 * "The Love Car Displacement" (S4E13): Sheldon, Leonard, Howard, Raj, Penny, Bernadette, and Amy make a weekend trip to a spa/four-star hotel because of a science conference that is going on. Sheldon uses this once on Raj, after Penny enters his and Leonard's hotel room and he feels no need to stay in there if they will be having "coitus".
 * "The Thespian Catalyst" (S4E14): Sheldon teaches a class horribly. Amy suggests that teaching is a performance art (requiring the ability to communicate, entertain, and engage), and that Sheldon could be a better teacher if he took acting lessons. Sheldon uses his knock twice, both for Penny (Once to ask Penny to give him acting lessons. The second time while dramatically saying "Penny?", when Penny opens the door to ask what is wrong, Sheldon replies, "Nothing. I was acting").
 * "The Toast Derivation" (S4E17): Leonard, Howard, and Raj spend all their leisure time at the apartment where Raj and Priya are, instead of spending time at Sheldon and Leonard's apartment. Sheldon "teaches" Leonard a lesson by hosting a social gathering at Sheldon and Leonard's place, inviting Kripke, Zack, Stuart, and LeVar Burton. Sheldon uses his knock once for all his friends at Priya and Raj's apartment.
 * "The Prestidigitation Approximation" (S4E18): Howard shows Sheldon a card trick, and Sheldon cannot figure out how the magic trick was done. Sheldon uses his knock once for Leonard, Sheldon wanted to practice figuring out how the card trick works while Leonard and Priya are in bed.
 * "The Agreement Dissection" (S4E21): Priya finds legal loopholes in Sheldon and Leonard's Roommate Agreement. Sheldon uses his knock twice for Penny (Once so that he can use Penny's bathroom. The second time to tell Penny that the mean Indian lady tried to make Sheldon eat lamb).
 * "The Infestation Hypothesis" (S5E2): Penny gets an abandoned chair for her apartment and Sheldon believes its unsantiary. Sheldon uses his knock a total of six times in this episode, once for in a nightmare with Penny, four times in real life with Penny, and once with Amy. His last knock with Amy, she states that his obsessive knocking is indicitanct of obsessive-compulsive disorder, which Sheldon outright denies.
 * "The Isolation Permutation" (S5E8): Bernadette and Penny go wedding dress shopping without Amy, upsetting Amy. Leonard uses this knock on Amy to force Sheldon to talk to her.
 * "The Ornithophobia Diffusion" (S5E9): A bird won't leave Sheldon and Leonard's window sill. Sheldon uses this knock on the window to make the bird go away.
 * "The Flaming Spittoon Acquisition" (S5E10): Stuart asks Amy out on a date. Sheldon attempts to make Amy jealous by asking Penny out on a date. Sheldon uses this knock three times. Once for Penny, once for Amy to end her date with Stuart, and once for Penny, Bernadette and Amy (while the three are playing Twister).
 * "The Vacation Solution" (S5E16): University President Seibert forces Sheldon to take vacation. Sheldon prefers to spend his vacation time in Amy's lab and he uses his knock once in there.
 * "The Weekend Vortex" (S5E19): Amy is going to a family party, but Sheldon doesn't want to go. Sheldon uses his knock for Penny to give her gummy bears.
 * "The Launch Acceleration" (S5E23): NASA calls Howard to inform him the mission has been cancelled. Howard prepares for his upcoming wedding. Amy wants to take her relationship with Sheldon to a higher level. NASA calls Howard to let him know that the mission will proceed as planned after all. Sheldon uses his knock once for Amy (before their date night in which Amy would dip into her neurobiological bag of tricks to seduce Sheldon with the Super Mario Brothers video game music, pink Quik, and spaghetti with little pieces of hot dog cut up inside).