The Real Reason Behind This Sheldon Quirk From The Big Bang Theory

Sheldon Cooper's genius is well established on The Big Bang Theory, but he's also a man with plenty of quirks. Whether it's guarding his spot on the couch or sticking to his carefully crafted bathroom routine, the physicist is all about maintaining an orderly life. And that extends to always knocking three times before entering someone's room or home.

Throughout the show's 12 seasons, Sheldon's knocking led to plenty of hilarious exchanges, particularly between him and Penny. But his practice of always knocking three times is actually rooted in a traumatic moment from his past.

In the season 10 episode "The Hot Tub Contamination," Sheldon reveals to Penny that he once came home from college early expecting to find his house empty, but instead he heard noises coming from his parents' room. When he opened the door, he found his father with another woman. "It's also why I never open a door without knocking three times. I mean, the first one's traditional, but two and three are for people to get their pants on," he explained to his friend.

Seeing Sheldon open up to Penny about the origins of his knocking practices was one of The Big Bang Theory's most emotional moments, and the event that the physicist described may soon lead to a meaty storyline on the spin-off series Young Sheldon, as well.

Weirdly, Young Sheldon has deviated from the older Sheldon's account of his childhood on more than one occasion since it began. The spin-off's continuity errors are particularly glaring since both shows are run by members of the same creative team. However, George Sr.'s affair seems far too important to the Cooper family's story to be skipped over entirely.

So far, Sheldon's dad hasn't shown any signs of having an affair, but the tension between him and his wife Mary has been growing with each season. In the season 4 episode "A Musty Crypt and a Stick to Pee On," the couple's disconnect is highlighted even more during a road trip. Still, showrunner Steve Molaro is reluctant to commit to having George's affair play out on the show.

"I don't know what we're going to do about that. I'm aware of it, I know it exists. I also know that... Adult Sheldon may not want to tell the audience that story," Molaro said of the affair in an interview with TVLine. "So maybe it happened, maybe it didn't. That doesn't mean we need to see it. Also, maybe there's a path where what he thought happened then, and what he finds out now, [is] different. I don't know... It's an interesting area of the show."

It's understandable why Molaro is protective of George Sr. — after all, Sheldon's father is a richly drawn character in his own right on Young Sheldon. However, to ignore the affair entirely would be a misstep — not only because it would offer up a dramatic story for the show to explore, but also because it was such a pivotal moment in Sheldon's young adulthood that affected him deeply for the rest of his life.