Soft Kitty is a song that Sheldon's mother used to sing to him when he was sick as a child. Today, he still requests his caretaker to sing this song to him whenever he falls ill, though it has become more of a Penny-and-Sheldon thing, because they are the only two characters that are ever shown singing this song to each other (besides Sheldon's mother to Sheldon).
In S02E21 The Vegas Renormalization, Penny had to sing "Soft Kitty" to Sheldon again on the pretense that "...Homesick is a type of being sick", since he had to spend the night in Penny's apartment because he locked himself out of his and Leonard's.
When Sheldon was feeling sad about being betrayed by his friends (and not being able to attend Comic Con) in S03E01 The Electric Can Opener Fluctuation, Penny starts singing "Soft Kitty" in an attempt to cheer him up, but was interrupted by Sheldon as he said, "That's for when I'm sick!! Sad is not sick."
In S03E08 The Adhesive Duck Deficiency, when Penny was a little lightheaded from the powerful pain medication and a muscle relaxer that the hospital gave her after she dislocated her shoulder from slipping in the bathtub, Penny asked Sheldon to sing "Soft Kitty" to her. After much prompting, the two sing it as a round.
At the end of S04E02 The Cruciferous Vegetable Amplification, Penny sings Soft Kitty to Sheldon, or more accurately, Sheldon's Mobile Virtual Presence Device (Sheldon in robot form) after Sheldon falls down the stairs. Sheldon plays "Soft Kitty" on his recorder (woodwind musical instrument) while Penny sings "Soft Kitty".
In S05E06 The Rhinitis Revelation, Sheldon's mom, Mary, sings "Soft Kitty" to Sheldon after he gets sick from being in the rain.
Little ball of fur.
Purr, purr, purr.
- In November 2010, The Big Bang Theory held a contest where contestants uploaded their version of Soft Kitty, and other users could vote for their favorite.
- While referred to on the show as "Soft Kitty" the actual song is called "Warm Kitty". Lyrics were written by Edith Newlin, the tune is traditional. There is also a version by Patsy Biscoe.
- When sung to Sheldon, "Soft Kitty" has never been shown to finish without being interupted and Sheldon demanding it be sung "from the top".