God’s announcement of a child to be born was met with various responses throughout Scripture.  Sarah, Abraham’s wife, laughed (Genesis 18:9-15).  Zechariah doubted (Luke 1:18).  By contrast, Mary submitted.  She believed the angel’s words and agreed to bear the child, even under humanly impossible circumstances.  God is able to do the impossible.  Our response to his demands should not be laughter or doubt, but willing acceptance.