For twelve seasons, The Big Bang Theory has chronicled the lives of a group of friends -- Sheldon, Leonard, Howard, and Raj -- and their achievements in their scientific work at Caltech and their adventures in love and nerd fun outside the university. Over the course of the series, some of the main characters have found love and have been paired off with each other. Leonard got the girl of his dreams in neighbor Penny, Sheldon eventually fell in love with fellow scientist Amy, and Howard and Bernadette have been married for over half of the series' run, with two children running around.

As much as the series is about the mad cap adventures of this quirky group, it's also come to be defined by the relationships within its core cast of characters, whether these be romantic or friendship. Arguably one of the best relationships to come out of the series has been the unlikely friendship between the ultra genius Sheldon and the decidedly non-academic Penny. While they began the series as adversaries, due to their totally different ways of life, Sheldon and Penny have come to have one of the strongest friendships in the entire series, producing countless heartwarming and hilarious moments over time.

While the show has tended to focus more on the romances in recent years, to the detriment of some of its original dynamics, Sheldon and Penny's friendship still remains one of the best decisions The Big Bang Theory has ever made.

Here are the 20 Wild Revelations About Sheldon And Penny's Relationship.

She is one of the only people allowed to sing "Soft Kitty"

If there's one thing that can be said of Dr. Sheldon Cooper, it's this: he's an absolute creature of habit. One of the recurring behaviors that Sheldon has displayed over the series is his need to be serenaded with his childhood favorite song "Soft Kitty" whenever he's sick, and only a select few people are allowed to sing this song to him.

In addition to his family members, who have sung the song for him his entire life, Penny quickly becomes one of the unsuspecting few who are allowed the privilege (or duty) of singing to him. What's even more surprising is that Sheldon even sang Penny "Soft Kitty" once, too, when she had injured herself and he was taking care of her.

She's also one of the only people allowed to hug him

Sheldon has never exactly been the warmest and fuzziest member of the group. He struggles with expressing his emotions and public displays of affection were, for a very long time, unthinkable for him. Enter Penny, who is basically the human embodiment of sunshine and affection, and it was only a matter of time before she wore down his defenses.

In the season two episode "The Bath Item Gift Hypothesis", Penny surprises Sheldon with Leonard Nimoy's autograph for a Christmas present, and Sheldon -- entirely beside himself -- hugs her. It's one of the first signs of real emotion from Sheldon in the entire series, and only the first of many mutual hugs that the duo share.

They have a strong romantic fanbase that has dubbed them "Shenny"

Boy meets girl. Girl meets boy. It doesn't take much more than that for the good people of the internet to become interested in the idea of pairing characters together in romantic situations, no matter how unlikely the reality of that outcome may be. Sheldon and Penny have been paired together romantically by eagle-eyed fans from the very beginning of the series.

It's not hard to see why. In the pilot alone, Penny and Sheldon have some cute moments that could be read as flirting, including Penny calling Sheldon a "Beautiful Mind genius guy" and Sheldon shyly responding. Beyond that, their banter has provided some of the most enjoyable moments in the entire show, so it's only natural that people would flock to their dynamic. Thus, the fan relationship known as Shenny was born.

Beyond being frenemies, they work wonderfully as a team, too

Sheldon and Penny's early relationship was best characterized as one between adversaries or frenemies. They rarely ever saw eye to eye and it made for some great comedic moments. However, a few times during these fraught early years, the two were able to find common ground and become unlikely allies as coworkers and mentor-student. In the season two episode "The Barbarian Sublimation", Penny becomes addicted to an online game and Sheldon serves as her mentor.

"The Work Song Nanocluster" also finds Sheldon and Penny working as a team in order to produce Penny Blossoms, a cutesy hair accessory Penny is selling online. "The Gothowitz Deviation" finds Sheldon attempting to improve some of Penny's behavior through the use of positive reinforcement via chocolate. Also, in "The Gorilla Experiment", the truly unthinkable happens, as Sheldon manages to teach Penny some basic physics.

Sheldon is more supportive of her acting dreams than her boyfriend is

The characters' careers on The Big Bang Theory play a big role in the overarching plot of the series, seeing as the majority of them are scientists. However, few characters have had the sort of career path that Penny has had. When the series began, Penny was an aspiring actress who wound up getting a job at the Cheesecake Factory, and there she stayed for much of the early seasons, securing the occasional acting job along the way.

Yet, while Penny's passion for the arts may never have wavered, the support she's received in the series -- especially from Leonard, her eventual husband -- has been sporadic at best. However, one person who has remained surprisingly consistent in his support of her acting has been Sheldon, who even once commented that Penny is a remarkable actress after seeing her perform onstage.

She has taught him a lot about life and love

Simply put, Sheldon Cooper is a snob. If a person isn't skilled in the sciences or smart in traditionally academic senses, he will go out of his way to malign them at any turn. Penny is included in this category. However, no matter how different Penny and Sheldon may be in their levels of intellect, Penny has still found some surprising ways to teach Sheldon some important lessons over the years.

While Sheldon is occasionally able to teach Penny things that she never learned in school, Penny is able to teach Sheldon about life and love, such as how to handle issues in relationships and how to be more open with his emotions. Also, if there's an occasional pop culture lesson thrown in along the way, all the better.

The series has teased their romance on more than one occasion

It's become increasingly common practice on television in recent years -- particularly in the rise of social media -- for series to bait fans along with teases of romantic relationships that will never actually happen. For The Big Bang Theory, that relationship they just love to tease is, of course, Sheldon and Penny's.

There have been two fantasy dream sequences about the pair in romantic situations -- one in which Penny attempts to sleep with Sheldon, and the other in which the two kiss passionately right in front of Leonard. There has also been an actual scene that took place that found Penny tackling Sheldon to the ground and kissing him all over his face. However, of course, none of that led to anything but fan hopes being fed and quashed all at once.

Amy was introduced just as Sheldon and Penny's friendship had become a real fixture in the series

The third season episode "The Adhesive Duck Deficiency" represented a key turning point not only in Sheldon and Penny's relationship, but also in fan and media response to the would-be relationship. After dislocating her shoulder, Penny finds herself taken care of by Sheldon, who even breaks his behavioral patterns and sings "Soft Kitty" to her. The physical humor and emotional growth between the characters is particularly off the charts in this episode, and fans were hopeful that this could finally be the beginning of more for them.

However, later that season, following an upward trajectory of screentime and closeness between the onetime frenemies, the series abruptly introduced a new female lead -- Amy Farrah Fowler. The curious timing of Amy's introduction has often led to fans wondering whether she wasn't just created for the purpose of steering fans away from the Sheldon-Penny bond.

Their friendship was sidelined as his relationship with Amy developed

As the series progressed, it only seemed more and more like early fan hesitance regarding the introduction of Amy's character was well-founded. While many of the series' early seasons contained quasi bottle episodes that have become known in some corners as "Sheldon and Penny episodes," those episodes have been almost entirely absent ever since Sheldon and Amy began a romantic relationship.

If anything, the series has trended towards producing "Leonard and Penny episodes" and "Sheldon and Amy episodes." Sheldon and Penny's friendship is still thrown the occasional bone beyond a moment or two of banter, but the dynamic that was once the defining part of the series is hardly anywhere to be found in comparison to what it once was.

Kaley Cuoco isn't a fan of the idea of Sheldon and Penny as a couple

Actors are often asked for their thoughts on storylines that are going on in their series, so it was really only a matter of time before Kaley Cuoco was approached for her thoughts on the dynamic between Sheldon and Penny -- and, even more pertinent, the idea that some fans have wanted them to become a romantic couple. For her part, Cuoco isn't a fan of the idea.

"What’s so funny is that Penny’s really become a mother to Sheldon. ... His character is like a child and it’s funny how much a part of each other’s lives they’ve become," she reflected in 2010, before offering an even more damning assessment. "I think [people] love [the idea] because it’s so not realistic. It’s like another planet. It’s not going to happen, so they want it to happen. And [if it did happen], they wouldn’t want it anymore."

However, Jim Parsons totally believes the idea of them romantically

While Kaley Cuoco may never have really understood what fans were expecting out of the idea of a Sheldon and Penny romance, it seems like Jim Parsons has been understanding of the idea from the very beginning. When reflecting on filming one of Sheldon and Penny's dream sequence kiss scenes, Parsons admitted, "Very early on working on this show, I began working a lot with Kaley on a lot of different scenes because Penny and Sheldon had a lot of scenes together, and it was a very easy relationship to be a part of on-camera. Obviously, as different as she and I are — certainly as different as the characters are — that may not seem like the most logical thing to say, but as performers, she and I work very easily together."

Further confirming his understanding of the duo as a romantic one, he noted, "The first time we rehearsed [the kiss], it was literally just like an extension of the verbal dance we've been doing for eight and a half years."

She is one of the only people he trusts to drive him around

Sheldon is both a creature of habit and a creature of considerable amounts of loud, picky opinions. If he doesn't like something, he won't hesitate to let you know. If you're not doing it the right way, he won't just tell you you're doing it wrong -- he'll go out of his way to tell you the right way, which is, of course, his preferred way of doing things. Few characters were ever allowed the "privilege" of driving Sheldon around in the early seasons of the series.

In recent years, he has become a little more flexible in his transportation, though he seldom sits anywhere other than the passenger's seat. However, in the early days of the series, there were only two characters who were allowed the joy of chauffeuring him around: Leonard, his put upon best friend and roommate, and Penny.

The laundry room is sort of their spot

The Big Bang Theory doesn't make use of many complicated sets. With the bulk of its action taking place in apartments, offices, or the Caltech cafeteria, there isn't a whole lot of unfamiliar territory in the series at this point. One set piece that was used far more frequently in earlier seasons, and in conjunction with Sheldon and Penny moments, was the laundry room.

While not seen very often in recent seasons as their relationship has taken a back seat, this particular set served as their place, for lack of a better term. Whether they were engaging in a prank war with each other, exchanging an unlikely hug, or discussing relationship issues, Sheldon and Penny often found each other for an important moment in the laundry room.

Entertainment Weekly declared them the best part of the show

We've already addressed the fact that Sheldon and Penny's relationship has provided the series with some of its most unexpectedly enjoyable moments, but we've yet to really say that not only do they provide the series with some much needed levity and warmth, but their development also just so happens to be one of the best written parts of the show on the whole.

We're not the only ones who think so. In a review of an early series episode, Entertainment Weekly said it pretty much perfectly: "If it wasn’t already abundantly clear, last night’s Big Bang Theory locked down this iron-clad axiom: Sheldon + Penny > Practically anything else on the show. This is no knock on Leonard, Howard, or poor Raj, who was especially MIA last night, and especially missed."

She has occasionally sat in his spot

Few things irritate Sheldon Cooper the way someone sitting in his claimed spot of preference on the couch in the main apartment does. Sheldon has explained time and again why he prefers to sit in the same spot on the couch -- it is his ideal location in the apartment, especially in relation to the television.

There has so far only been one character that Sheldon consciously lets sit in his designated spot -- his grandmother, Meemaw. However, on a few occasions, Penny has been allowed to sit there without any sign of protest from Sheldon. Perhaps it was just a continuity error on the series' part, but it makes for a sweet detail nonetheless.

Jim Parsons and Kaley Cuoco are incredibly close

When a series runs for as long as The Big Bang Theory has, it's only natural that cast member relationships either strengthen or strain over time. In the case of Jim Parsons and Kaley Cuoco, the talented actors behind Sheldon and Penny, it really does seem that their friendship was strong from the very beginning, and has only grown stronger with time.

Their respective social media accounts are filled with selfies and set photos of the two of them goofing around and commenting enthusiastically on each other's posts. Also, when Parsons won a Golden Globe for his portrayal of Sheldon at the 68th Annual Awards, Cuoco was the presenter of the award, and her unbridled glee included squeals of joy and enthusiastic hugs for her costar, showing that their bond has become just as warm and special as that of their on screen counterparts.

His ultra conservative, judgmental mother adores her

Through Young Sheldon, the universe of The Big Bang Theory has begun to show Sheldon's ultra strict mother, Mary Cooper, in a much more sympathetic light. In her appearances on The Big Bang Theory, however, Mary is often entirely critical and judgmental of everyone she comes into contact with, providing a source of conflict for Sheldon and his friends, even when she usually means well enough and just wants to show her love for her son.

One character who she has been surprisingly fond of, however, is Penny. Although she doesn't necessarily approve of Penny's much more modern female lifestyle, Mary has always seemed to get along with Penny, which just further reaffirms the importance of Penny in Sheldon's life.

Many of the series' bloopers occur because Kaley Cuoco can't help but laugh at Jim Parsons

We've already talked about how close Kaley Cuoco and Jim Parsons have become over the years. The show is all the luckier for that fact, too, because when actors don't get along, it's sometimes really obvious on screen, and makes it impossible to enjoy dynamics that were once key parts of a series.

However, an added benefit for the show as a result of Cuoco and Parsons' closeness? The sheer abundance of hilarious bloopers that fans can enjoy that often occur because Cuoco has a hard time keeping it together whenever she's working with Parsons. It may make producing the series harder, since scenes have to be reset, but the adorably hilarious bloopers left behind make it all worth it.

Leonard and Amy are both jealous of their closeness

Sheldon and Penny may never have been really treated in any romantic way by the series, but that hasn't prevented their respective romantic significant others -- Amy and Leonard -- from displaying their jealousy at the duo's bond over the years. At various points early in the series, Penny is seen slipping out of Sheldon's bedroom after talking with him, caught by a very confused and jealous Leonard.

In the season seven episode "The Anything Can Happen Resurgence", Amy admits that she is jealous and wary of the closeness Penny and Sheldon have, which she and Sheldon have never had despite how long they have been together. The season nine episode "The Viewing Party Combustion" similarly finds both Leonard and Amy complaining about the inexplicable close bond that Sheldon and Penny have, and how their closeness often leads to the two of them discussing things secretly behind their significant others' backs.

She inspires him in surprising ways

Penny really doesn't know the first thing about science, but sometimes, a true outsider can provide the most unlikely, and most helpful form of inspiration of them all. In the season 11 episode "The Solo Oscillation", Sheldon sets out to work on scientific projects all by himself without Amy's interference, only to find himself talking to Penny about them.

It's in the process of talking with Penny that, suddenly, Sheldon finds himself faced with the potential of a great scientific discovery. While scientists have been working for decades on the idea of string theory, Penny, in her naiveté, makes an offhanded remark about strings having knots that causes Sheldon to wonder if maybe there could be an entire new dimension to string theory scientists had never considered.

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What's your favorite part of the relationship between The Big Bang Theory's Sheldon and Penny? Let us know in the comments!