On The Big Bang Theory, Penny is the outsider of the group. She hasn't gone to college, she is a struggling actress, and has quite an interesting past. She contrasts very much with her book-smart, socially awkward, and nerdy friends. Having said that, she becomes a defacto mother for the group, supporting and taking care of them if necessary.
One of Penny's most prominent features is her sarcastic sense of humor. She is not afraid to insult the boys to their faces about their eccentricities, their relationships, and even their careers. Basically anything about them. Not to mention, she is whip-smart. Just take a look at these zingers if fans don't believe it.
"What's Up, Buttercup? What's The Word, Hummingbird? What's The Gist, Physicist?"
Penny's dry sense of humor and delivery of her quips is what drives most of her humor, especially in her interactions with Sheldon. In the second episode of season 5, Penny and Sheldon get into an argument over a chair that she had found outside.
Sheldon tries on several occasions to get Penny to get rid of the chair and, each time, Penny says the above words. Not only is the creative rhyming hilarious in itself, but her deadpan delivery to Sheldon is the icing on the cake.
"Oh, This Isn't Alcohol. This Is A Magic Potion That Makes Me Like You."
In season 6, after a girl's trip to Vegas goes wrong, Penny, Bernadette, and Amy join the boys for a game of D&D. When Sheldon warns Penny that they don't drink alcohol during the game, Penny dryly tells him the above sentence.
It is a running joke throughout the whole series that Sheldon mocks Penny for her lack of knowledge in all "nerdy" matters. And Penny is not shy about retaliating in kind about how much she "doesn't like him." This is an example of her protecting her dignity using humor.
"Yeah, Well Your Ken Can Kiss My Barbie."
Sheldon is very particular about many aspects of his life, including his seat on the couch and his food. Unfortunately, Penny is not so so considerate of this fact. When she ends up getting three "strikes" from Sheldon, the two go to war.
She interferes with his laundry night and he tells her that she is "dealing with forces beyond her ken". She replies with the above quote. Although Penny isn't the most educated out of the group, she demonstrates her quick wit by using his own vocabulary against him.
"Okay, Were You Talking Before She Got Upset? [...] That's Probably It."
As the series goes on, Sheldon makes a lot of improvements in terms of his social cues and empathy. However, it's not always enough, especially for Penny. When Sheldon comes to her for advice about Amy getting upset, she tells him the above quip.
Penny does care about Sheldon deeply, but it is still very easy for her to tease him about his lack of "normalcy." She is always quick to display her surprise that Sheldon is in a healthy relationship at all.
"Sweetie, Every Night You Don't Kill Him In His Sleep, He Wins."
When Leonard discovers a DVD that Sheldon rented that should have been returned years ago, Sheldon decides to school Leonard in empathy by having him wear an itchy sweater until they can return it. Leonard breaks down to Penny, and she tells him the above quote to make him feel better.
The roommate dynamic between Sheldon and Leonard, by all accounts, is baffling to most viewers. However, it does make it very easy for Penny to make comments about it at Sheldon's expense.
"Don't You Need To Stay Here So The Mother-Ship Can Find You When It Returns?"
After Sheldon and Leonard's apartment is robbed, Sheldon becomes paranoid and starts investing in new security measures. He even begins considering moving. Penny, who spent a difficult night dealing with Sheldon's paranoia, asks him if the mothership would be able to find him if he leaves.
This is one of Penny's sharper quips towards Sheldon. Even after all their years of friendship, she still cannot fathom how his mind works, especially when she ends up suffering because of it. Comparing him to an alien is very common.
"Awww, He's Got The Same Look My Little Nephew Gets When He Can't Figure Out How I Got His Nose."
There are few things that Sheldon Cooper struggles to understand, but they do exist. When he struggles to figure out Howard's card magic trick, Penny makes the above comment, all too happily.
Sheldon's arrogance is a prominent aspect of his personality and he frequently uses it against Penny. That is why, whenever she can, Penny takes the opportunity to turn the tables. And given that Sheldon's is always bragging about his accomplishments and his intelligence, Penny has no issues infantilizing him.
"His Right Hand Is Calling Him?"
Penny is many things, including ruthless, especially towards those that annoy her. In season 2, when Howard's girlfriend, Leslie Winkle, calls him, Penny leans over to Leonard and asks him this question, implying that his girlfriend is...not real.
Penny acts disdainful towards Howard in earlier seasons of the series, especially when he incessantly tries to flirt with her. She is also very aware of how much he struggles in terms of romance. Her completely serious delivery makes the line that much funnier.
"So That Means, You're A Doctor, You're A Doctor, You're A Doctor, You're A Doctor And ... Howard, You Know A Lot Of Doctors."
The educational status within the group is one of the many points of contention (as well as running gag) within the group. Howard is the only one without a Ph.D., and Penny happily will join in the fun against him. When it is revealed that Bernadette is getting her Ph.D., Penny makes the above joke.
Although it is a pretty hypocritical thing for Penny to say considering she didn't to college, she knows exactly how to hit below the belt. And her passive-aggressive, exaggerated laughter is the cherry on top.
" If You Let Her Go, There Is No Way You Can Find Anyone Else. Speaking On Behalf Of All Women, It Is Not Going To Happen. We Had A Meeting!"
When Howard and Bernadette get engaged, her father demands that the couple sign a prenup. When Howard expresses doubts about it, Penny lays down the law by reminding him of how lucky he is.
It's a pretty cruel line on Penny's part for insinuating that Howard will be miserable without Bernadette. However, years of Howard's failed flirting with her and his difficulties with women plays a big part in their friendship dynamic.