Sheldon Cooper finally revealed in The Big Bang Theory Season 10 the mystery behind his three door knocks.

Fans of TBBT knon that before he enters a door, Sheldon (Jim Parsons) knocks three times while saying the name of the person he is looking for. He, however, never explained why he does such thing. We thought this might be just one of his weird habits until The Big Bang Theory Season 10 Episode 5.

In the The Hot Tub Contamination episode, Penny (Kaley Cuoco) decided to take Sheldon out of the apartment since he and Amy (Mayim Bialik) are struggling to compromise on his cohabitation rules. In an ice cream parlor, Sheldon confessed to Penny why he knocks three times. The Caltech genius said it started when he was 13 years old and on spring break from college. He said he went home early when his professors ran out of math to teach him. When he got home, he though their house was empty as his mother was at a Bible study. However, he heard noises coming from his parents’ bedroom. When he opened the door, Sheldon saw his father having sex with a woman who was not his mother.

“It’s why I never open a door without knocking three times. The first one’s traditional, but two and three are for people to get their pants on,” Sheldon explained to Penny.

Poor Sheldon was traumatized. Sheldon said his father knew that he saw everything but they never spoke about it. He also never told his mom about it. From then on, Sheldon declares himself first before entering a door to make sure he will never witness the same situation again.

Watch Sheldon’s revelation here.

Sheldon also admitted to Penny his concern that he might end up hurting Amy like what his father did to his mother. He concluded that he might as well start hitting on women since he is going to be unfaithful like his father. Penny pointed out how silly Sheldon’s logic is and told him to be more concerned with his manners as he is hurting Amy right now.

Back at home, Sheldon apologized to Amy for his bathroom schedule and told her that he is now ready to take their relationship to the next level.

The Big Bang Theory Season 10 moves to its regular time on Thursdays at 8/7c starting Oct. 27 on CBS.

Don’t miss more of Sheldon and Amy’s co-habitation experiment in The Big Bang Theory Season 10.