Soft Kitty is a song that Sheldon's mother used to sing to Sheldon when he was sick as a child. Today, he still requests his caretaker to sing this song to him whenever he falls ill. Sometimes he also sings to others who have fallen ill.
In "The Vegas Renormalization" (S02E21), 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 "The Electric Can Opener Fluctuation" (S03E01), 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 "The Adhesive Duck Deficiency" (S03E08), 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 "The Cruciferous Vegetable Amplification" (S04E02), 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) as a round while Penny sings "Soft Kitty".
In "The Rhinitis Revelation" (S05E06), Sheldon's mom, Mary, sings "Soft Kitty" to Sheldon after he gets sick from being in the rain after he felt that his mother was ignoring him over spending time with his friends.
In "The Werewolf Transformation" (S05E18), Penny mentions to Sheldon that she sings "Soft Kitty" to him when he's sick.
This song's tune originates from nineteenth century Warsaw (Poland) and it was used in a lullaby titled "Wlazł kotek na płotek", written by Oskar Kolberg and composed by Stanisław Moniuszko. However the Lyrics for this lullaby were about a kitten on a fence.
The modern song which we know now is in fact named "Warm Kitty". The lyrics were written by Edith Newlin and the tune is traditional. It was published in 1937 by Willis Music Co. These lyrics differ slightly to the one used in the show, with the show changing the sequence.
||Warm Kitty (original song): Warm kitty,: Soft kitty,: Little ball of fluff.: Sleepy kitty,: Pretty kitty,: 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.
- When sung to Sheldon, "Soft Kitty" has never been shown to finish without being interrupted and Sheldon demanding for it to be sung "from the top".
- In "The Fish Guts Displacement" (S06E10), when Sheldon rubs Vicks on Amy's chest, it sounds like he is humming "Soft Kitty" as the scene ends.
- When Kaley Cuoco broke her leg while horse riding in 2010, the cast and crew of The Big Bang Theory made her a "Get Well Soon" video where they all sang "Soft Kitty".
- Wil Wheaton sang "Soft Kitty" for his wife Anne when she was home sick with bronchitis and sinusitis.
- In 2014, the various renditions featured in the show, performed by Kaley Cuoco, Laurie Metcalf and Jim Parsons as their respective characters, were remixed together for a commercial for the show.