Have you ever wondered why Sheldon Cooper knocks three times?

Sheldon Cooper, played by Jim Parsons, has some unique habits, but one of the most distinctive is his knocking pattern. Whenever Sheldon knocks on a door, he doesn’t just knock once; he knocks three times, calls out the person’s name, and repeats this pattern three times. But why this specific ritual?

In Season 10, Episode 5 of the Big Bang Theory, Sheldon reveals his secret to Penny. When Sheldon was 13 years old, he walked into his house that was supposed to be empty. Then he heard a sound that was coming from his parents’ bedroom. He knocked once and [a video fragment of Sheldon saying a part of the story about seeing his father with another woman.
After this event, Sheldon adopted this knocking habit as a coping mechanism…However, there’s much more to this story.

In Young Sheldon, we actually live through this moment together with Sheldon played by Iain Armitage. In season 7, episode 4, Sheldon returns home and walks in on what he thinks is his father cheating. It turns out, he actually saw his own mother, dressed up, trying to surprise his dad.

For all this time Sheldon has been thinking that his father cheated on his mother. However, it was just a big misunderstanding that they have never discussed.

The Big Bang Theory (TV Series 2007–2019) and its spinoff series Young Sheldon (TV Series 2017– 2024, the young Sheldon portrayed by Iain Armitage).