The song Be Careful, It's My Heart is performed by Irving Berlin .
<lengthy instrumental intro>
Be careful, it's my heart
It's not my watch you're holding, it's my heart
It's not the note that I sent you that you quickly burned
It's not the book I lent you that you'll never return
Remember, it's my heart
The heart with which so willingly I part
It's yours to take, to keep or break
But please, before you start
Be careful, it's my heart