Leonard (Johnny Galecki) and Sheldon (Jim Parsons) captured the hearts of millions when CBS' "The Big Bang Theory" premiered in 2007. The series has become one of the most successful sitcoms in TV history and will reach a milestone when the latest episode airs this week. 

On Thursday, Feb. 25, the show, which is in it's ninth season, will air it's 200th episode, titled "The Celebration Experimentation." Episode 17 will have guest star Adam West and the gang will be celebrating the birthday of Sheldon. Other past characters are also expected to pop up and return for the special installment. Before you watch the 200th episode of the series, check out these interesting facts about "The Big Bang Theory":

  1. In the theme song "The Big Bang Theory," by the Barenaked Ladies, there's a line that says "Neanderthals developed tools." This line is actually false as Homo habilis actually created tools.
  2. Sheldon and Leonard were named in honor of television producer Sheldon Leonard.
  3. It has never been revealed what Penny's last name is. She is also the only member of the gang whose mother hasn't appeared.
  4. According to the Huffington Post, Sheldon's catchphrase "Bazinga" was started by writer Stephen Engel, who used to yell out the phrase when he'd play pranks on the other writers.
  5. CBS gave the show consecutive three season renewals, which was the first time in television history that's ever been done.
  6. Actress Mayim Bialik  actually has a Ph.D. in real life. In 2008, she earned a Ph.D. in neural biology, meaning she's just as smart as her character Amy Farrah Fowler.
  7. Executive producer Bill Prady got the idea for Raj (Kunal Nayyar) being unable to speak to woman unless he's had alcohol from a former co-worker at a computer company he worked at. 
  8. The way Bernadette speaks on the show is not actually how actress Melissa Rauch talks. Rauch based the high-pitched voice off of her mother.
  9. The elevator scenes on the show are filmed on the same floor. The floor gets redecorated each time, to make it appears as though the characters are moving up different levels to their apartments.

 The 200th episode of "The Big Bang Theory" airs Thursday on CBS at 8 p.m. EST.