Penny sings "Soft Kitty, Warm Kitty" to Sheldon in a few episodes of "The Big Bang Theory". In at least one other episode, Sheldon's mom sings "Soft Kitty, Warm Kitty" to him but with a slightly different melody. Here is Penny's version, and here is Sheldon's mom's version. Were the differences a choice of the actors, the writers, or the director, and was it done for a specific reason (possibly to show how the two women "mother" Sheldon a little differently)?
To be musically specific, Penny sings the first 3 notes as separated by a minor third, and the next 3 notes as separated by a major third. Sheldon's mom (Mary Cooper) sings the first 3 notes separated by an octave, and the next 3 notes separated by a minor 7th. The same thing repeats in the second verse for each of them.