The Big Bang Theory: 10 Things You Completely Missed From Season One
8. Sheldon's Knock
At times, Sheldon Cooper feels less like a person and more like a bundle of quirks. His obsessive compulsive disorder is such a defining part of who he is, any casual fan can rattle off a couple of his Sheldisms.
Of course, his most recognised ritual is The Sheldon Knock. When he's about to enter a person's home or office, Sheldon will rapidly tap on the door three times, saying the owner's name in-between each knock, before inviting himself in.
But this trait wasn't always set in stone since he doesn't not follow this rule in Season 1. In the second episode, Sheldon upsets Penny and is forced to apologise to her. When he goes up to her apartment, he politely knocks on the door only one time. Instead of saying Penny's name, he grumbles, "I have a master's and two PhDs. I should not have to do this."
In the fifth episode, he knocks on her door once and quietly waits to be invited in.