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"Warm Kitty" is a children's song, popularized by the characters Sheldon and Penny in the American sitcom The Big Bang Theory where it is rendered as "Soft Kitty". The words to "Warm Kitty" were written by Edith Newlin, and are still under copyright. The tune was taken from a traditional English children's song, "Little bird, have you heard..." but the note values had to be changed to fit the new words. The tune is also very closely related to the German folk song "Hänschen klein".
In the The Big Bang Theory, the lyrics are "Soft Kitty, Warm Kitty, little ball of fur, Happy Kitty, Sleepy Kitty, purr purr, purr". This is sung by Sheldon, Penny, and Sheldon's mother. They sing it to each other when one of them is sick.