Soft Kitty is a song that the mother of Sheldon, a character from the Big Bang Theory, use to sing to him whenever he was sick.
The song was first introduced in The Pancake Batter Anomaly, where Penny had to sing it to Sheldon when he caught a cold. In The Vegas Renormalization, Penny had to sing soft kitty to Sheldon again on the pretense "...Homesick is a type of being sick". 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, Penny starts singing Soft Kitty, but was interrupted by Sheldon as he said "That's for when I'm sick! Sad is not sick." In The Adhesive Duck Deficiency, when Penny was a little lightheaded from powerful painkillers she asked Sheldon to sing her Soft Kitty as a round. At the end of The Cruciferous Vegetable Amplification, Penny sings Soft Kitty to Sheldon after he falls down the stairs.
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.