Monday's episode of The Big Bang Theory will reveal how Jim Parson's character Sheldon got into the habit of knocking three times before entering any room.
The episode is titled The Hot Tub Contamination and it will begin with Amy and Sheldon complaining about finding it difficult to live together. They start fighting about Sheldon's bathroom schedule and it escalates, leaving them wondering if they did the right thing by moving in together.
Warning: This post contains spoilers from the upcoming episode of The Big Bang Theory
Later, while talking to Penny about the problems they face as a couple, Sheldon tells her an incident from his childhood that led to his habit of knocking on the door three times.
Here's what happens, according to The Big Bang Theory wikia:
He came home from college early for spring break when he was 12 because the college had run out of math to teach him. His mother was at Bible Study, and he was expecting the house to be empty. He heard a sound from his parent's bedroom, and he opened the door to see his father in bed with another woman. They locked eyes, he shut the door and ran, and they never ever mentioned it to each other.
According to Sheldon, the first knock is for tradition and the other two knocks are to warn others to pull up their pants if they are in a compromising situation.
Elsewhere in the episode, viewers will see Bernadette and Howard deciding to take a small vacation before the baby comes, and Raj and Stuart bonding in Howard's hot tub.
The Big Bang Theory airs Mondays on CBS.