"Soft Kitty" is a children's 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 feels ill. Sometimes, he also sings it 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 apartment.
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 (according to her, "Injured and drugged is a kind of sick"). 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 Recollection Dissipation" (S10E20), Sheldon is recovering from both stress and a fever brought on by him working on two different projects at once. Amy sings the song to him in three different languages (English, German, and Mandarin) while playing an autoharp.
In "The Werewolf Transformation" (S05E18), Penny mentions to Sheldon that she sings "Soft Kitty" to him when he's sick.
In "A Sneeze, Detention, and Sissy Spacek" (S01E13), when Sheldon is sick in bed, Mary sings the song to him.
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.
Both "Wlazł kotek na płotek" and "Soft Kitty" are about cats, the similarities ends there, the melodies doesn't sound alike.
Soft kitty, Warm kitty, Little ball of fur, happy kitty, sleepy 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.
- The daughters of the original songwriter filed a lawsuit on the show in December 2015 for using the altered version of the song without permission. Because of that, the song has not been performed after Season 8. The lawsuit charges have since been dropped; however, the song remained seldom used.