An Internal Laugh

When Avrohom confronts Sarah as to why she laughed about bearing a son, Sarah denies the matter and says "I didn't laugh."  The Ramban asks how could it be that a great prophetess like Sarah would deny that what Hashem told Avrohom?  Furthermore, how would Sarah lie to Avrohom in any circumstance?  My great-grandfather explains that possuk 12 says vatizchak Sarah b'kerbah, Sarah laughed internally.  The laugh of Sarah was an emotion deep down in her depths, it was so deep even she didn't realize it and therefore denied the episode.  The mussar is clear.  Sometimes a person will feel that they are doing the right thing but deep down they no its wrong.  One must be in tune with their inner feelings to see what is driving their actions.