‘Big Bang Theory’ Season 10 Spoilers: Story Behind Sheldon’s 3-Knock Quirk Revealed In Episode 5 [RECAP]

“Big Bang Theory” Season 10, episode 5 saw Sheldon (Jim Parsons) and Amy (Mayim Bialik) struggling to adjust as they share living space with each other for the first time. After an explosive argument over his strict bathroom guidelines, the latest episode of the CBS series also revealed the secret behind one of Sheldon’s most notable quirks: his three-knock habit.

In “The Big Bang Theory” Season 10, episode 5, titled “The Hot Tub Contamination,” Amy makes it known that she doesn’t appreciate Sheldon’s controlling nature. Her boyfriend, on the other hand, gets angry that she won’t follow his rules like his last roommate Leonard (Johnny Galecki) did. When Sheldon threatens to find a new roommate, Penny (Kaley Cuoco) and Leonard makes sure to gloat, “No backsies.”

When Sheldon and Amy fail to resolve things between them, Penny and Leonard decide to step in and help them communicate better with each other. However, Sheldon refuses everything that Amy wants to do, like store their toothbrushes in the same holder like other couples, and even threatens to break up with her.

Unable to find a common ground, Penny takes Sheldon for a drive to think things over and cool off. Sheldon complains about Amy’s shortcomings the whole time, and Penny finds that she can relate because her boyfriend has the same issues. They lament over a number of flaws their partners have, such as Amy and Leonard eyeing them like they’re pieces of meat instead of appreciating their minds. On Leonard’s end, he and Amy have a similar conversation about Penny and Sheldon.

Meanwhile, Howard (Simon Helberg) bids Raj (Kunal Nayyar) and Stuart (Kevin Sussman) goodbye as he and Bernadette (Melissa Rauch) have plans to head to Palm Springs for a babymoon. But their trip doesn’t push through when Bernadette’s morning sickness worsens. Howard suggests that they just stay at home instead and not tell their friends.

However, they get a shock upon reaching their home when they find Stuart in their hot tub. The situation gets even more comical when Raj pops in, gets into the hot tub and is scared out of his mind when Stuart pops out of the water.

Raj tells Stuart that he’s single now. Bernadette feels bad that they didn’t know Raj had broken up with his girlfriend. But Howard reminds her that their friends are freeloading off them, drinking their wine and enjoying their hot tub when they’re not home. They finally decide to surprise Raj and Stuart when the latter refuses to leave the tub because he’s been naked the whole time.

Back to Sheldon's apartment, Sheldon reminds Amy of all the good things she’s done in her boyfriend’s life and how she’s helped him change for the better. He encourages her not to give up on Sheldon now and to continue helping him become a real boy.

Penny, on the other hand, takes Sheldon to an ice cream parlor where she listens to him explain why he wants to find a new girlfriend. Sheldon tells her that when he was 13, he came home from college and found his father in bed with another woman in his parents’ bedroom. At the time, he’d locked eyes with his father, ran to his own bedroom and never spoke about the incident afterward. This is also the reason why he’s developed a habit of knocking three times before entering a room.

Sheldon doesn’t want him and Amy to turn into his parents, who argue and fight all the time. Penny suggests that he try to mend things with Amy rather than jumping the gun regarding their relationship. In the end, Sheldon agrees to make things right with Amy, so they go back to the apartment. Sheldon decides to do so by scrapping the bathroom schedule altogether and telling Amy that he was never interested in finding a new girlfriend. To prove that he means it, he asks Amy to share a toothbrush holder with him, making Penny tear up.

Find out what happens next when “The Big Bang Theory” Season 10, episode 6 airs Monday, Oct. 24 at 8 p.m. ET on CBS.