Knockknockknock...

...Amy? Penny? Sometimes even, Sheldon?

The sidelong joke began in the series as just another of Sheldon Cooper’s OCD habits—two episodes ago, he even realized in a full-on drunken stupor he only knocked on Will Wheaton’s house twice—and evolved into a huge hallmark of the Big Bang Theory series.

(Fun fact: on more than one early occasion, the number of knocks was actually four. The fans remember three, more rhythmically, and three is the number featured on many a t-shirt!)

To celebrate the classic parallel to the original brand of humor, a brief overview of the more classic “knock-knock-jokes.”

Reasons why Sheldon’s knocked on Penny’s door (usually during crude waking hours):

Penny: “Sheldon, what is it?”

Sheldon: “Leonard’s asleep.”

Penny: “…Thanks for the update.”

Sheldon: “Penny. Penny. Penny.”

Penny: “What?”

Sheldon: “This is banana bread.”

Penny: -pointing to the door- “This is a doorknob.”

Sheldon: “Penny. Penny--”

Penny: “What?”

Sheldon: -knocks one more succession- “…Penny.”

Sheldon: -after checking his watch- “Penny. Penny. Penny.”

Penny: Opens the door. Sheldon braces himself.

Penny: “…Sheldon.”

Sheldon: “It’s 11 a.m.”

Penny: “I know. You’re safe.”

(Don’t you just love continuity?)

Sheldon: -knocks- “Penny--”

Penny: -knocks from her side- “Sheldon.”

Sheldon: “…Penny.”

Penny: “Sheldon.”

Sheldon: “Penny.”

Penny: “Sheldon?”

Sheldon: “Penny!”

Penny: “Sheldon!”

And who can forget the episode where Penny slips in the shower and hurt her arm? One of the more touching (figuratively) and realistic episodes: it showed that Sheldon really can be called upon in times of crisis, even if it takes him six knocks, and the beginnings of a bathroom-door knock, to do so.

One automated computer voice, one Flash Gordon knock, and one apology knock later, Sheldon ended up cementing his relationship with Penny through routine after all; albeit not in the way even he expected. I’m sure the day Sheldon can walk freely into Penny’s (or, now, Amy’s, apartment) without the subsequent knocks would be a day the fans remember well.