The producer of The Big Bang Theory, Lee Aronsohn, revealed that Sheldon's famous habit of knocking three times was actually unscripted. Aronsohn came up with the idea during filming and it became a staple gag in the show, demonstrating Sheldon's obsessive-compulsive tendencies. Later on, the show revealed the dark origins of the habit, which added a humanizing touch to the character. In Young Sheldon, however, the show skipped over George's cheating storyline, which posed a challenge for the prequel. Towards the end of The Big Bang Theory, Sheldon's habit wasn't as prominently featured, possibly because he became more secure in his relationships.
