Sheldon's Knock

Sheldon only knocks in a certain way. He will always knock three times and then say the name(s) of people in the room, repeated thrice (e.g. *knock knock knock* Penny, *knock knock knock* Penny, *knock knock knock* Penny). His knock has gone through several changes before the familiar knock was created.

Origins and development
The earliest version of his knock was shown in The Staircase Implementation. At this time, Sheldon would knock 3 times and say the person's name. However, unlike his current knock, he would knock slowly and wait a few seconds before he would repeat the knock again. Early in the series, he knocked on doors normally, but the first version of his knock debuted in The Loobenfeld Decay. In this version of his knock, he would quickly knock 4 times and say the person's name. When using this knock, Sheldon would repeat his process continuously until he received a response. His current knock was first seen in The Euclid Alternative when Sheldon went to ask Penny for a ride to work. It has been his knock ever since then, with few exceptions in which he varied it throughout the show.

Sheldon's Knock used by others
Throughout the series, Sheldon's knock has been mimiced by other characters as either a parody or to make the person think it is Sheldon knocking.


 * In The Panty Piñata Polarization, Penny agrily mimics the knock when she confronts Sheldon over her missing laundry.
 * In The Vartabedian Conundrum, when Sheldon had thought he had lost his voice, he had used a computer generated voice and knocked at Penny's door.


 * In 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. He knew she wouldn't answer the door if she knew it was him.


 * In The Isolation Permutation, Leonard knocks on Amy's door like Sheldon in order to make Sheldon comfort her when she was upset.

Modifications of Sheldon's Knocks
There is a new use of Sheldon's knock in Season 5 Episode 10 - The Flaming Spittoon Acquisition - after he knocks three times, Penny yells from inside, "Who do we love?" Then Sheldon says "Penny" as he always does. She says this all 3 times he knocks.

In The Toast Derivation, when Sheldon goes to Raj's apartment and knocks, he says, "All my friends" 3 times between knocks.

In The Justice League Recombination, Sheldon (dressed as the Flash) knocks incredibly fast as well as saying Penny's name in time betwwen each knock: *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny *knock* Penny. When Penny opened and question what the hell was wrong with him, he answered "I'm the Flash, I just knocked 30,000 times".