He has a genius-level IQ of 187, but displays a fundamental lack of social skills, a tenuous understanding of humor, and difficulty recognizing irony and sarcasm in other people, although he often employs them himself.

Raj is unable to speak in front of women outside his own family due to selective mutism--unless he's drunk, in which case he says very inappropriate things.

In "The Bath Item Gift Hypothesis", Penny presents Sheldon with a napkin signed by Leonard Nimoy whom she met at the Cheesecake Factory. She apologizes that the napkin is dirty because he wiped his mouth on it. Completely overwhelmed, Sheldon exclaims that he now has the DNA of Leonard Nimoy. He briefly considers cloning Nimoy, but Penny denies Sheldon's request for a "healthy ovum".

Joyce Kim was a North Korean spy who attempted to obtain information on Leonard's work by seducing him. The relationship lasted only 27 days before Joyce defected back to North Korea.

Sheldon sees a certain elegant appeal to "sword fighting". It's indoors, no running, no throwing, no catching and no gym shorts that get pulled down or up.

In "The Re-Entry Minimization", Howard returns from his five-month stay on the space station expecting a hero's welcome but gets a rude awakening when it's simply Bernadette. Those chants of "Howie! Howie! Howie!" in the background? That's for Howie Mandel, who also arrived at the airport -- albeit not from space.

In "The Luminous Fish Effect", Sheldon's mother says, "I tell you, I love the boy to death but he has been difficult since he fell outta me at the K-Mart."

Sheldon has restraining orders from his heroes Leonard Nimoy, Carl Sagan, and Stan Lee, as well as television scientist Bill Nye.

When Leonard enters the lab of geeky girl scientist Leslie Winkle, she is heating up a Cup o' Noodles with her laser. He asks her to a bio-social exploration with a neuro-chemical overlay as the modification of our colleague/friendship paradigm, with the addition of a date-like component. Instead of the date, Leslie proposes that they go straight to kissing.

Although Sheldon briefly abandons his research in string theory to study dark matter, he returns to string theory in the eleventh season and works with Amy to develop a Super-Asymmetry Theory which eventually puts them in contention for a possible Nobel Prize.