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Sheldon's Biggest Secret On The Big Bang Theory

It's no secret that Sheldon Cooper (Jim Parsons) in "The Big Bang Theory" is a man of many eccentricities. Overly intellectual, incapable of comprehending irony, and subject to any number of odd hygienic, social, or academic behaviors, Sheldon is practically the poster boy for unrealistically awkward nerds. One of Sheldon's most frustrating qualities is that he is almost incapable of keeping someone else's secret, no matter how private it is. For example, in Season 2 he revealed that Penny (Kaley Cuoco) had never gone to community college as she claimed.

Despite this, even Sheldon isn't a completely open book. There are a few things about himself that he will only share with those closest to him. In fact, he's taken the time to chart out the various tiers of information and the people that are privy to that knowledge in his "Zones of Privacy" diagram.

Sheldon will also keep secrets if it suits his best interest, like when he neglected to inform his friends that he had a driver's license for two years because he likes being "chauffeured" around. However, that's small pickings compared to Sheldon's biggest, most traumatic secret on "The Big Bang Theory."

Sheldon once walked in on his father having an affair

Anything dealing with romance and relationships is far from Sheldon's forte, but he manages to make things work with the equally intellectual Amy (Mayim Bialik), who he eventually marries. While they were still dating, however, Sheldon's poor romantic skills caused problems on multiple occasions. In the Season 5 episode "The Hot Tub Contamination," shortly after the couple first moved in together, the two bickered so badly that Sheldon left to go pick up women at an ice cream shop. 

After getting their ice cream, Penny (who chauffeured Sheldon on this occasion) asked him what his real issue was. Sheldon reveals that since he and Amy are arguing, he must prepare her for when his natural male urges compel him to cheat on her. However, he refuses to actually talk to any women at the shop. Finally backed into a corner, Sheldon reveals his big secret.

When Sheldon was 13, he once walked in on his father having an affair with another woman while his mother was out of the house. Traumatized by the experience, Sheldon ostensibly tries to save Amy from heartbreak by sabotaging things now — hence his quest to pick up girls at an ice cream shop. Since then, Sheldon always knocks three times before entering a room so that "people have time to put their pants on." Luckily, Penny snapped him out of it and Sheldon was able to return home and make amends with Amy.