“Why.” The Hebrew has a demonstrative pronoun (“this, such”) after “Why,” as if to say, “Why this laughter,” or “Why such laughter.” Thus the Hebrew expresses God’s astonishment that Sarah would laugh, after all, He created the heavens and the earth. So, with Sarah listening, God expresses his astonishment to Abraham and says in essence (as it is well expressed in the NET text note): “Why on earth would Sarah laugh?” Once we see God’s expression of astonishment we can better see why Sarah was afraid and denied it, especially after God goes on to say, “Is anything too hard for Yahweh?”