One Major Big Bang Theory Mystery Has Finally Been Explained

Spoilers for Monday's episode of The Big Bang Theory below!

Of all the characters on CBS' The Big Bang Theory, Sheldon Cooper is the most mysterious while also being an open book. He has a wide variety of specific likes and dislikes, but we hardly know why he's so particular about most things. Well, now we have the reason behind one of his more noticeable quirks.

Sheldon's three tier knock is a famous gag on The Big Bang Theory. Instead of knocking once and waiting to see if the door is answered before trying again, Sheldon has always knocked three times in a row, while saying the name of the person he's looking for in between each knock.

Sheldon has a lot of habits that are just plain odd, from forcing a strict bathroom schedule on his roommates to only ever sitting in one spot on the couch and getting highly flustered if anyone else attempts to sit there, but tonight's Big Bang Theory finally revealed that Sheldon's obsessive need to knock three times in a row comes from a really honest and, frankly, shocking place.

As we know by now, Sheldon and Amy began their experiment in living together at Penny's place last week. Amy's apartment is out of commission for a whole five weeks, and Sheldon accepted the challenge of living with his lady love while Penny finally gets a chance to live with her husband Leonard in their, finally Sheldonless, apartment. Anyone who's watched more than a couple of Big Bang episodes knows that living with Sheldon has many challenges where his, often odd, behavior is concerned, and, this week, Amy was having trouble getting him to compromise on some of the rules he has on co-habitating. After Amy suggests that they share the same toothbrush holder, Sheldon announces that he wants to see other people. Penny them takes him out of the apartment so that he and Amy can both calm down, and this is when Sheldon makes a startling confession to her.

While sitting in an ice cream parlor, Sheldon finally reveals why he knocks the way he does. When he was 13-years-old and on Spring Break from college, he went home early one day when the professors ran out of math to teach him. When Sheldon got home, he heard noises coming from his parents' bedroom and opened the door to find his father having sex with a woman who was not his mother. Damn. So, that's where this comes from...

You guys, poor little Sheldon was traumatized. Sheldon also mentioned that his father realized he was there, but they never spoke about it and he never told his mom. That's a pretty big secret for someone like Sheldon to hold on to. And, now that we know this, it's not impossible to imagine that some of his other quirks stem from the same incident somehow. What we learned here is that if that situation doesn't make you want to be absolutely sure you're getting what you're looking for before you open a door, nothing else will. Penny can never be annoyed by Sheldon's knocking again.

We can all see what's in store for Sheldon when The Big Bang Theory airs Mondays on CBS.