Sheldon Cooper is one of The Big Bang Theory's most appreciated characters, due to his sharp wit and well-timed comebacks. His inability to empathize with other people results in countless acerbic comebacks, some of which are intended to be sharp, and others that are accidentally hilarious and not intended to hurt others.
As the self-appointed group leader, Sheldon considers himself to be above criticism, but he is quick to criticize others, often making barbs about the other character's work or personal lives. He is especially quick-witted when challenged, and his exchanges range from comical to openly insulting, depending on his mood.
When He Can't Resist Belittling Leonard's Work
Sheldon is judgmental of all the characters, but he is especially harsh on Penny, making countless remarks throughout the series that imply that Penny is intellectually inferior to the rest of the group. When Penny approaches Sheldon and asks him to teach her about Leonard's work, Sheldon takes the opportunity to treat Penny as a test subject, likening her to a gorilla that was successfully taught sign language in another experiment.
Sheldon's lesson begins with a lecture about ancient Greece, resulting in Penny becoming frustrated with Sheldon's long-winded lesson. She asks Sheldon to just explain what Leonard is working on. Upon learning about his work, Penny says that Leonard's work doesn't seem that complicated, to which Sheldon replies, "It's not. That's why Leonard does it."
When Donkey Kong Gets Personal
Whilst working as a pharmaceutical sales rep, Penny finds herself the object of a doctor's affection. When the would-be Romeo arrives at Penny's apartment, having found her address online, there's an awkward moment of realization as the doctor becomes aware that although Penny flirts with clients to make sales, she is actually engaged to Leonard.
Leonard, Sheldon, Howard and Raj end up at the doctor's home, where they explore the doctor's nerdish games room. When Leonard sees the Donkey Kong game, he is transported back to his childhood and tells Sheldon that Donkey Kong was his game when he was a child. Sheldon retorts, "Because it's a story of a pretty blonde girl tirelessly pursued by a small, oddly-shaped man?"
When He Insults The Judge
Following a mishap that results in Sheldon issued a fine for running through a red light, he ends up in court. As Penny was involved in the incident, Sheldon brings her along to court as a witness, but prepares a script for her. He believes that Penny is a "loose cannon" who can't be trusted to speak without saying the wrong thing.
Ultimately it's Sheldon who says the wrong things. In true Sheldon style, he insults the judge. "Good morning, your honor. Dr. Sheldon Cooper appearing in pro se - that is to say, representing himself," he begins. The judge tells Sheldon that he knows what "pro se" means, as he went to law school. "Yet you wound up in traffic court," retorts Sheldon.
When He Brings Howard Back Down To Earth
When Bernadette tells Howard that she isn't interested in having a casual relationship, Howard panics and wonders if Bernadette is really the best woman he can attract. Totally oblivious to the unrealistic expectations he has of his future spouse, he tells the group that he wants a Megan Fox lookalike, and doesn't want to "settle."
The group disagrees with Howard, and Penny in particular rejects his views. Sheldon agrees with Howard, though. He says that Howard is right to seek the best possible mate to carry on his genes. Howard thanks Sheldon, and Sheldon continues. "And whether that propagation is in the interest of humanity is, of course, an entirely different question."
When He Has To Choose Between Amy Or Video Games
When Raj suggests that all the men have a video game night akin to the all-nighters they had before everyone got girlfriends, Sheldon is excited. However, Amy reminds him that the two of them are supposed to attend her aunt's birthday party. Sheldon tries to acquiesce but ultimately decides that video games are preferable to a family get-together.
Although intelligent, Sheldon sometimes exhibits childlike qualities and can't always determine how his actions impact other people. This is especially true in his relationship with Amy, who tells Sheldon that her aunt will be disappointed if Sheldon doesn't attend the party. "She's 93," Sheldon replies. "She won't be disappointed for very long."
Leonard, Stephany & Sheldon
When Howard meets Stephany at a club and tries to impress her with the Mars Rover, things quickly go awry and he calls his friends for help to steal the security footage that shows his role in the Rover getting stuck. When Stephany and Leonard meet, the chemistry is strong and they start dating, much to Sheldon's delight, as he adores Stephany.
Sheldon is concerned that Leonard will do something to ruin his relationship with Stephany, and he takes steps to try to keep them together. "Of the handful of women Leonard's been involved with, she's the only one I have ever found tolerable," Sheldon explains. "What about me?" Penny asks, to which Sheldon retorts, "The statement stands for itself."
When He Mocks Zack About The Starfish
When everyone realizes that Penny is seeing her ex-boyfriend, Zack, the group proceeds to mock Zack's lack of intelligence, amusing themselves at his expense. Although Zack tries to fit in with the group, he doesn't succeed. "You know," he tells the group, "I saw this great thing on the Discovery Channel. Turns out that if you kill a starfish, it'll just come back to life."
Zack's starfish comment is too much for Sheldon to ignore. "Was the starfish wearing boxer shorts?" He asks. "Because you might have been watching Nickelodeon." Although friendly and possessing an apparently genuine interest in science, it's not surprising that Zack didn't fit in with the group.
When He Gets French Toast On Oatmeal Day
When Penny makes French toast for breakfast on oatmeal day, Sheldon is upset about the change in his routine. He becomes indignant about the incident, and despite admitting that the French toast smells good, he throws it in the garbage, as it's oatmeal day.
Leonard tells Sheldon that he needs to find a better way to interact with Penny. "You catch more flies with honey than with vinegar," Leonard tells him, referencing Sheldon's behavior. Sheldon replies, "You can catch even more flies with manure. What's your point?" Leonard's advice didn't fall on deaf ears, however, as Sheldon soon began "training" Penny by giving her chocolates for displaying what Sheldon deemed acceptable behavior.
When He Has The Card Trick Meltdown
When Howard demonstrates a card trick, Sheldon tries to figure out how the trick is accomplished, but cannot. As everyone else seems to work out what the trick is, Sheldon becomes increasingly frustrated. He is especially upset when Penny, who he considers to be much less intelligent than himself, understands the trick.
Sheldon is often curt with Penny, but his patience with her is noticeably shorter when he is frustrated by something. In one exchange, Penny tells Sheldon, "Not knowing is part of the fun." Sheldon muses over this comment. "'Not knowing is part of the fun'," he repeats, before saying caustically, "Was that the motto of your community college?"
When He Mocks Leonard About His Imaginary Babies
When Penny moves into the apartment building, it's love at first sight for Leonard. He goes out of his way to make Penny feel welcome, even going so far as to try to retrieve Penny's television from her ex-boyfriend's apartment. The endeavor doesn't go well, and Penny's ex-boyfriend robs both Sheldon and Leonard of their pants.
Lovesick Leonard utters the now-famous line, "Our babies will be smart and beautiful," as he wistfully watches Penny from the hallway. Sheldon, less romantic than Leonard, says, "Not to mention, imaginary." In the final episode of the series, Sheldon mentions the exchange in his Nobel Prize acceptance speech, before letting it slip that Penny is pregnant.