Sheldon: Can you sing Soft Kitty?
Penny: What?
Sheldon: My mom used to sing it to me when I was sick.
Penny: I'm sorry, honey, I don't know it.
Sheldon: I'll teach you. [sings] "Soft kitty, warm kitty, little ball of fur, happy kitty, sleepy kitty, purr purr purr." Now you.
Penny: [sighs] [sings] Soft kitty, warm kitty…
Sheldon: Little ball of fur. Keep rubbing.
Penny: Little ball of fur.