Soft Kitty

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, or sings it to others who are ill.

Appearances
The song was first introduced in S01E11 The Pancake Batter Anomaly, where Penny had to sing it to Sheldon when he caught a cold.

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 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. Sheldon plays "Soft Kitty" on his (woodwind musical instrument) as a round 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 after he felt that his mother was ignoring him over spending time with his friends.

In The Proton Resurgence, Sheldon sings the song to Arthur Jeffries while the latter is in hospital.

Origins
This song originates from XIX century Warsaw (Poland) and it was used as a lullaby. Original lyrics for this song were written by. What is interesting...as Sheldon would say, is that the music for this song was written by famous   in year  at a party with his friends as three voice canon.]

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Lyrics Soft kitty, Warm kitty, Little ball of fur. Happy kitty, Sleepy kitty, Purr, purr, purr.

Trivia

 * 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 "". 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".
 * Wil Wheaton sang "Soft Kitty" for his wife Anne when she was home sick with bronchitis and sinusitis.
 * In "The Fish Guts Displacement", when Sheldon runs the Vicks on Amy's chest, it sounds like he is humming Soft Kitty as the scene ends.