Before Kaley Cuoco was solving airport-related murders in “The Flight Attendant” and antagonizing Batman in “Harley Quinn,” the actress was well known for playing Penny in the CBS sitcom “The Big Bang Theory.” In the series pilot, Cuoco’s character is the new neighbor to physicists Leonard Hofstadter (Johnny Galecki) and Sheldon Cooper (Jim Parsons), becoming a fish out of water in their world of science, fandom, and social awkwardness. 

The Pancake Batter Anomaly

Cuoco’s early seasons as Penny quickly established many memorable moments for the character, particularly in her relationship with Parsons’ breakout character. Penny and Sheldon get a chance to spend some alone time together in the 11th episode of Season 1, “The Pancake Batter Anomaly,” in which Sheldon somehow manages to catch a bug, throwing him into a panic. Knowing to keep their distance from Sheldon when he’s sick, Leonard, Raj, and Howard escape to a “Planet of the Apes” movie marathon, leaving Penny to take care of Sheldon for the entire episode. It’s a great way to introduce the dynamic of these characters, with Penny begrudgingly giving in to Sheldon’s various demands, such as rubbing VapoRub on his chest and, most notably, singing a song his mother used to serenade him with when he was sick as a child. 

That song, “Soft Kitty, Warm Kitty,” eventually became a recurring joke throughout “The Big Bang Theory,” with Penny singing it to Sheldon or Sheldon singing it to others when they need comfort. However, its origin is in this episode, where Cuoco completely nails the performance of Penny slowly realizing why Leonard, Howard, and Raj were so quick to abandon Sheldon when he got sick.

The Tangerine Factor

The will-they-won’t-they between Penny and Leonard drives most of the first season, as Leonard’s crush on his neighbor steadily grows. It eventually comes to a head in “The Tangerine Factor,” which is also the first season finale of “The Big Bang Theory.” The episode opens with Penny stewing after a recent breakup, with Leonard debating whether or not he should go over to her apartment to help her.

When a frustrated Penny wonders why she can’t ever go out with a nice guy, Leonard finally makes his move and asks Penny to go out with him. Penny’s nervousness about the date also results in a hilarious and memorable moment when she asks Sheldon for advice, and Sheldon compares their prospective relationship to the Schrödinger’s Cat paradox, which Penny struggles to understand.

Their respective unease eventually climaxes when Leonard shows up at Penny’s apartment for their date, and the two kiss for the first time. In one of Cuoco’s funniest moments in the show’s first season, the kiss ends and Penny, finally understanding the paradox, declares “the cat’s alive,” before the couple leaves for their first date. 

The Barbarian Sublimation

By the time the second season of “The Big Bang Theory” rolled around, the characters and their dynamics with each other had been established enough where comedy could be earned by turning them on their heads. This was the concept behind “The Barbarian Sublimation,” where Sheldon introduces Penny to “Age of Conan,” an MMORPG video game.

Very quickly, Penny becomes obsessed with the video game, prompting her to annoy Sheldon with her various questions. It’s a fun reversal of Penny and Sheldon’s dynamics, given that Sheldon is often intruding on Penny’s life by incessantly knocking on her door. After convincing Leonard to try to reason with Penny, Leonard finds the only way he can get through to her is through the video game, though Penny responds to his attempt by chopping his video game character’s head off.

The ending of the episode, however, is one of Kaley Cuoco’s funniest moments. Dressed in sweatpants and covered in food stains after playing “Age of Conan” for hours on end, she accepts an offer to go out for a drink with Howard’s in-game character, which is the only wake-up call she needs to realize that playing this game for so long is unhealthy. 

The Panty Piñata Polarization

While many previous episodes of “The Big Bang Theory” showcase how Sheldon and Penny make unlikely friends, other episodes depict them as mortal enemies destined to take each other down. Their conflict in “The Panty Piñata Polarization” comes to a head when Sheldon bans Penny from their apartment after she eats his onion rings and sits in his favorite spot on the couch. After Penny refuses to take Sheldon’s order at The Cheesecake Factory, the two neighbors declare war on each other.

First, Sheldon denies Penny access to their apartment’s Wi-Fi, but Penny hits back in a major way: She takes up all the laundry machines on Sheldon’s “Laundry Day.” Sheldon responds by removing Penny’s laundry from the laundry room and leaving it to dry on telephone wires.

The final straw in Penny and Sheldon’s war against each other comes when Penny drags Sheldon’s mom into the affair. Sheldon receives a call from her insisting he apologize to Penny for how he’s been treating her, and he subsequently returns Penny’s laundry and revokes his decision to banish her from their apartment. Cuoco proves in this episode that while Penny may not have the scientific intelligence of her neighbors, she’s still very much capable of outsmarting them at their own game. 

The Bath Item Gift Hypothesis

Penny and Sheldon have also had their share of endearing moments as friends. In one of the show’s most iconic episodes, Season 2’s “The Bath Item Gift Hypothesis,” Sheldon is put into a stressful situation when Penny announces that she got him a Christmas gift. Sheldon becomes determined to buy her something of equal value, attempting to cover his bases by selecting several bath item gift baskets of varying prices. 

By the end of the episode, even the studio audience is shocked to discover what Penny got for Sheldon. Though he initially seems unimpressed by a napkin from the Cheesecake Factory, Sheldon soon discovers that it’s been used and signed by Leonard Nimoy, who visited the restaurant during one of Penny’s shifts. Sheldon, ecstatic that he can now grow his own Leonard Nimoy, gifts Penny all of the gift baskets he bought but, fearing that’s not enough, Sheldon actually hugs Penny. 

Meanwhile, Penny also spends the episode getting to know David Underhill, a fellow physicist played by Michael Trucco. Leonard is sent into a spiral of his own over this, considering he and Penny broke up due to her insecurity at feeling less smart than him, before the two of them discover that David is actually married, giving Cuoco an opportunity to act Penny’s heartbreak alongside her meaningful moment with Sheldon. 

The Work Song Nanocluster

“The Work Song Nanocluster” is another opportunity for Penny and Sheldon to come together with a common goal. At the start of the episode, Sheldon discovers that Penny is starting her own business, selling custom-made flower barrettes she calls “Penny Blossoms.” Sheldon decides to start helping Penny streamline her workflow, recruiting the others to help as well. 

To their surprise, they receive an order for 1,000 Penny Blossoms with a tight deadline, thanks to the website Leonard created with the option for “One Day Rush,” forcing them all to hunker down and get the Penny Blossoms made within the day. This also leads to a moment where Sheldon drinks coffee for the first time, giving him the idea to don a Flash outfit as he speedily helps them make the Penny Blossoms. 

Though the episode is a simplistic bottle episode only featuring the main five cast members, it’s a good showcase of the friend group’s willingness to help Penny. Cuoco also once again shines in her interactions opposite Parsons, in what The AV Club called in their positive review of the episode, “another of what’s become a string of smartly written, masterfully performed dialogue scenes on this show.” 

The Jiminy Conjecture

By the start of season 3, Leonard and Penny’s relationship has been restarted, but things aren’t going so well for them in the second episode, “The Jiminy Conjecture.” After Penny overhears Leonard describing their sex as “just fine,” the two have an argument that results in them deciding to get drunk and then have sex to see if that improves. Over the course of the episode, Leonard and Penny get increasingly more and more drunk, while Sheldon, Raj, and Howard have their own separate adventure.

It’s a great opportunity to showcase the chemistry between Cuoco and Galecki three seasons in, especially given that the two actors were dating in real life when this episode was filmed and aired. Unfortunately, Leonard and Penny’s plan goes awry as the two end up spending the night throwing up in Penny’s bathroom rather than having sex as they had planned. 

Thankfully, the two characters are able to mend fences after their night of drinking and puking, ending the episode by deciding to stay friends for a few short seconds before they resume making out. In episodes like this, Cuoco’s charismatic on-screen relationship with Galecki might be more intense, but it’s certainly still entertaining to watch unfold. 

The Adhesive Duck Deficiency

In a reversal of Season 1’s “The Pancake Batter Anomaly,” Season 3’s “The Adhesive Duck Deficiency” sees Sheldon tasked with the job of taking care of Penny. While Leonard, Raj, and Howard are camping without Sheldon, Penny accidentally slips in the shower and dislocates her shoulder, forcing Sheldon to come over and help her. It’s another classic episode where Penny and Sheldon’s dynamic is on full display, with perfect performances from Cuoco and Parsons to boot.

Penny’s shoulder dislocation results in some high-stakes moments between her and Sheldon, particularly when Penny needs Sheldon to drive her to the emergency room, even though Sheldon cannot drive. In reaction to Sheldon’s slowness, Penny quips at one point, “Warp speed ahead, Mr. Spock,” which showcases that despite having no interest in the group’s nerdiness, Penny has learned more than she’s willing to admit about “Star Trek.”

The real kicker at the end of the episode is, after their emergency room visit, Sheldon takes Penny back to her apartment, where she requests that Sheldon sing “Soft Kitty, Warm Kitty” to comfort her. It’s the first time in the show that Sheldon sings the song instead of Penny, but it makes for one of the sweetest moments between these two often adversarial character

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