In the show, the song "Soft Kitty" was described by Sheldon as a song sung by his mother when he was ill. Its repeated use in the series popularized the song. A scene depicting the origin of the song in Sheldon's childhood is depicted in an episode of Young Sheldon, which aired on February 1, 2018. It shows Sheldon's mother Mary singing the song to her son, who is suffering with the flu.
The men are excited that Bill Gates is visiting Penny's pharmaceutical company, although she says that she cannot bring them along. Sheldon believes that it's an April Fools' Day prank. Leonard sees what hotel Gates is staying at on Penny's schedule, so he, Raj and Howard meet him in the lobby the day before. Once Sheldon sees proof, the men twice send him to the wrong hotel as real pranks. Penny says Leonard can meet Gates for real, so he fakes being sick to avoid him. When Penny puts Gates on a video call, he recognizes Leonard from the previous night. Meanwhile, Amy is tired of Bernadette only talking about her children, so Bernadette uses her expanded parental brain to learn random science facts.
Amy and Howard begin collaborating on a neuroprosthetics project, upsetting Sheldon and Raj, who feel they are not spending as much time with them. When they seek solace with an annoyed Bernadette, she exploits the situation to trick Sheldon into doing Howard's chores. Penny successfully employs the approaches recommended in one of Bernadette's parenting books to deal with Sheldon, but Leonard believes she is coddling Sheldon too much. Sheldon and Raj resolve the issue of missing their significant others by spending time together.
While Johnny Galecki is putting down the comic books for good as he says goodbye to experimental physicist Leonard Hofstadter, he's poised to return to another iconic role one more with the expanded season two order of Roseanne spinoff The Conners coming to ABC this fall. Galecki reprised the role of David Healy, ex-husband to Sara Gilbert's Darlene, for a handful of episodes in the show's first season and the door was definitely left open for more. Despite Galecki telling TV Line in February that he wouldn't consider jumping in full-time—"I think I need a little bit of space from being a series regular again," he said—he's hoping to "visit as much as possible." And Gilbert, who serves as EP on the series, remains hopeful as well. "I know he loves doing the show. I love having him. We have a magical time together," she told the outlet in January. "I would anticipate that he would do some [episodes] but I don't want to speak for him."
Raj hooks up with a recently separated woman, Nell, after one of his lectures at the planetarium. Her husband, Oliver, confronts Raj at the planetarium and then breaks down over the end of his marriage. After comforting Oliver, Raj talks to Nell and convinces her to give her husband another chance. Meanwhile, after diving once again into string theory and sprawling his work across his and Amy's apartment, Sheldon approaches a reluctant Leonard and Penny and convinces them to let him use his old room to study for a three-day trial period with an agreement to be quiet and stay out of their way. Sheldon surprisingly sticks to this agreement, being a reasonably good guest, which shockingly frustrates Leonard to no end. After Leonard lets out his frustrations, Sheldon then renegotiates the guest tenancy agreement for an extra few days and resumes being his normal demanding self, much to Leonard's relief.
Kevin Sussman as Stuart Bloom (recurring seasons 2–5, 7, starring seasons 6, 8–12): A mild-mannered, under-confident owner of a comic book store. A competent artist, Stuart is a graduate of the prestigious Rhode Island School of Design, and though he is socially awkward he possesses slightly better social skills. Stuart implies he is in financial trouble and that the comic book store now also is his home. He is later invited to join the guys' group while Howard is in space. Stuart gets a new job caring for Howard's mother later. After Mrs. Wolowitz's death, Stuart continues to live in her home, along with Howard and Bernadette, until he finds a place of his own.
In the episode "The Launch Acceleration", Amy tries to use her "neurobiology bag of tricks" to increase the attraction between herself and Sheldon. In the final fifth-season episode "The Countdown Reflection", Sheldon takes Amy's hand as Howard is launched into space. In the sixth season first episode "The Date Night Variable", after a dinner in which Sheldon fails to live up to this expectation, Amy gives Sheldon an ultimatum that their relationship is over unless he tells her something from his heart. Amy accepts Sheldon's romantic speech even after learning that it is a line from the first Spider-Man movie. In "The Cooper/Kripke Inversion" Sheldon states that he has been working on his discomfort about physical contact and admits that "it's a possibility" that he could one day have sex with Amy. Amy is revealed to have similar feelings in "The Love Spell Potential". Sheldon explains that he never thought about intimacy with anyone before Amy.
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