By The Big Bang Theory remains one of the world's most popular TV shows. Fans can't get enough of Sheldon and his gang of geeky pals.
We've put together a collection of awesome facts about the sitcom that you might not have known, from the meaning behind Sheldon's shirts to the origins of 'Soft Kitty'.
1. We still don't know Penny's surname
Unlike the other characters, Penny is the only one whose surname has never been revealed. Reports claim that Kaley Cuoco prefers to keep this a mystery throughout the whole series.
Co-creator Bill Prady has said that the name will be revealed eventually, but producer Steve Molaro has said the opposite, so who knows. Unless of course her name is Penny Penny Penny, and Sheldon's knocks make more sense.
2. Actress Mayim Bialik has an actual neuroscience PhD in real life
Amy Farrah Fowler is one smart lady, but it turns out that actress Mayim Bialik is pretty darn smart too. In 2007, she earned a PhD in neuroscience studies. After filming Blossom as a child, she attended UCLA having been accepted at Harvard and Yale.
She also earned a BS degree in neuroscience, Hebrew studies and Jewish studies. Her dissertation was an investigation of hypothalamic activity in patients with Prader-Willi syndrome, titled 'Hypothalamic regulation in relation to maladaptive, obsessive-compulsive, affiliative, and satiety behaviors in Prader-Willi syndrome'. Woah.
3. The cast are also highly skilled musicians
Aside from being academic geeks, most of the characters are also talented musicians. To achieve this, three of the stars are actually highly skilled themselves.
Johnny Galecki has been a cellist from a young age, while Jim Parsons and Mayim Bialik learned how to play the theremin and the harp especially for the show.
4. It was completed ripped off in Belarus
An unofficial version of the show emerged in Belarus called The Theorists, complete with four scientists named Sheldon, Leo, Hovart and Raj, while a blonde waitress named Natasha lived next door.
Production was soon called off after the actors caught wind of the rip-off, as apparently they were completely unaware of the original.
5. There's a real bee named after Sheldon's catchphrase
In 2012, biologists discovered a new bee species in Brazil and named it the euglossa bazinga after Sheldon's favourite word.
The name was chosen in honour of the fictional scientist, with André Nemésio of the Universidade Federal de Uberlândia saying at the time: "Sheldon Cooper's favorite comic word 'bazinga', used by him when tricking somebody, was here chosen to represent the character. Euglossa bazinga has tricked us for some time due to its similarity to E ignita, what led us to use 'bazinga'."
Funnily enough, Sheldon is actually allergic to bees, so he might not be able to meet his new friend.
6. Jim Parsons has never seen Star Trek
Sheldon is the unofficial King of Geekdom, and is an expert on everything from Marvel Comics to Red Dwarf. However, Jim Parsons isn't quite as clued up in the cult world.
Parsons has never actually been a fan of any of Sheldon's favourite TV shows, particularly Doctor Who and Star Trek. He once told Time: "I'm very frank about certain things, I get asked a lot of science things and I'm like, 'You're kidding'. And even the pop culture… I've never seen Star Trek. I don't know Doctor Who."
7. A grapefruit helped create the word 'Bazinga'
Sheldon's favourite word 'bazinga' was first uttered in the season two finale, but it was created with the help of a silly prank.
Writer Stephen Engel used the word to say 'Gotcha' in a recurring backstage gag involving an old grapefruit in the writers' room. As the word kept getting used, it eventually made its way to the actual script shortly before filming the finale. "I'd open it up and Stephen would say, 'bazinga'," said producer Bill Prady recalled. "It was Stephen's word for 'gotcha'."
8. CBS gave it a three-season renewal. Twice.
In an act that's quite rare for US TV networks, the show was recommissioned for three more series by CBS in 2011.
It was then given another three-season renewal. This is the first time in modern TV history that a scripted series has been given back-to-back three-season renewals. It got a two-season renewal last time, which was a bummer.
9. The characters spoke about Mayim Bialik before she was on the show
Back in the first season, Raj, Leonard and Howard think of people to replace Sheldon on their physics team. Raj suggests the actress Mayim Bialik, saying: "You know who's apparently very smart, is the girl who played TV's Blossom. She got a PhD in neuroscience or something." Two seasons later, Bialik joined the show as Amy.
Meanwhile, one of Johnny Galecki's first roles was in Blossom as Bialik's love interest who takes her to a "make-out party". He also starred opposite Sara Gilbert (Leslie) and Laurie Metcalf (Sheldon's mother) in Roseanne.
10. The elevator scenes are filmed on just one floor
This one you may have guessed, but the three floors of the guys' flat is actually just the same set redecorated each time. This means, if you were to visit the fictional apartment block, you'd be disappointed.
For the sake of a recurring gag, the actors and crew have to recreate the scene to make each floor different each time, which is surely quite a headache. This is probably why we don't see this comedy tool used as much in recent series.
11. 'Soft Kitty' is an actual song
'Soft Kitty' is the only thing that can help Sheldon feel better on the rare occasions he is ill, but did you know that it exists outside the show? Sort of.
The song dates back to the 1930s, and was titled 'Warm Kitty' by Edith Newlin. Producer Bill Prady revealed that his daughter sang 'Soft Kitty' while at pre-school, after her teacher first heard it while working in Australia.
12. The Big Bang Theory might be about the apocalypse
A recent video from Cracked posed the idea that The Big Bang Theory is secretly all about the end of the world, and that Sheldon, Leonard, Raj and Howard are the modern day equivalents of Einstein and Tesla. However, due to living in the 21st century and being obsessed with girls and TV, they are not concentrating on the bigger picture.
The Barenaked Ladies' intro theme goes through history on a particular timeline, culminating with our heroes. Cracked ponders whether the show's name is due to the fact that the big bang is expanding to the point where one day, one of the four will cause the universe to have a 'big crunch', effectively a reboot where everybody dies. That would be quite the gruesome end.
However, the writers have actually said that they have "no master plan" for how to end the show. Showrunner Steve Molaro said he thinks "it's great that some characters have barely spoken and we are seven years in," and that he is "excited about the possibilities for the future".
13. Howard has only female toys
Out of all the gang, Howard would probably be the last to be considered a feminist, but his toy collection might say otherwise. Or maybe he's just really pervy.
His large collection of sci-fi and fantasy figurines are made entirely of female characters. His room includes models of Batgirl, Cheetah, Wonderman, and his only male toy seems to be Jabba the Hutt – but even that features Princess Leia beside him.
14. Penny was originally Katie
The original title of the show was called Lenny, Penny and Kenny. We're assuming this means Sheldon was originally named Kenny, which is just plain wrong.
Plus, the pilot episode didn't feature Kaley Cuoco Sweeting as Penny, in fact it had actress Amanda Walsh as a character named Katie. Sheldon and Leonard meet her crying on the pavement after breaking up with her boyfriend and being left homeless, and Leonard quickly invites her to stay with them.
However, Katie's tough exterior didn't sit well with test audiences, and she was replaced by Penny.
15. Nearly everyone has seen Penny naked
Despite being the first to flirt with her, Howard is the only one of the male characters not to see Penny naked at some time or another.
Leonard has obviously seen her naked many times, while Raj had a one-night-sort-of-stand and Sheldon helped Penny to the hospital after she fell in the shower.
16. The pilot had a completely different theme song
We all know and love the theme tune from the Barenaked Ladies now, but it was originally a completely different song in the pilot.
Thomas Dolby's 1980s tune 'She Blinded Me With Science' was used back then, but it was later used as a short MIDI version as Howard's ringtone in 'The Lizard-Spock Expansion' episode in 2008.
17. The writers used a neat Star Trek reference for Wil Wheaton's 'character'
Wil Wheaton appears as a recurring version of himself in the show, and references his role of Wesley Crusher in Star Trek: The Next Generation on many occasions.
For proper Star Trek geeks out there, Wil's house number in the show (1701), is the same as the hull number of the USS Enterprise (NCC 1701-D).
18. Amy was nearly played by Kate Micucci
Singer and actress Kate Micucci eventually joined the cast as Raj's timid girlfriend Lucy, but she originally auditioned for Amy.
Although she wasn't quite right for Sheldon's partner in awkward love, the producers kept her on the list for future parts, and she returned as Lucy. As part of comedy duo Garfunkel and Oates, their video for 'Present Face' featured Kunal Nayyar himself.
19. Sheldon's shirts sometimes convey his mood
If you're a big Green Lantern fan you probably already knew this, but for the rest of us, this is pretty cool.
Sheldon often wears different coloured shirts bearing the Lantern's logo, but each shirt also has a different emotion attached to it in the comic world. Red stands for rage, blue is hope, indigo is passion etc.
Whenever Sheldon is wearing one of these shirts, the chances are his emotion is going to be represented by the colour.
20. This is what the apartment really looks like
Have you ever wondered what the layout of the main characters' apartment looks like? The clever clogs' at Roomsketcher have made a full 3D version of it.
Quite how Penny was able to afford such a luxurious apartment on waitress wages for so many years is beyond us.
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