"The Celebration Experimentation" marked an important moment for "The Big Bang Theory," as it was the 200th episode of the show. Airing as the 17th episode of Season 9, most of the story takes place during Sheldon's 36th birthday party, which causes a lot of feelings to rise up as a result of his poor childhood experience with birthdays after sharing them with his twin sister, Missy. However, Amy insists they throw a party for him.
Despite Sheldon's various demands, Amy, Penny, and Bernadette do a great job organizing it, even inviting Adam West as a guest appearance. However, Sheldon panics upon entering and hides in the bathroom, resulting in the characters debating how to get him out. This is where the episode becomes an incredible showcase of Kaley Cuoco's evolution of Penny over nine seasons.
After entering the bathroom to comfort Sheldon, Penny endearingly tells him that everyone showed up because they love him, and that she'd be willing to spend his birthday party in the bathroom with him. This episode, especially this moment, shows what Cuoco was best at while playing Penny for 12 seasons: She may not have the brains or the nerdiness of the rest of the characters, but she has the heart and emotional intelligence they all need in their lives.
