1) Sarah asks, “Can I have a child with such an old husband?” Was Sarah questioning G-d’s ability to do a miracle?
2) G-d asks Avraham why Sarah questioned whether she can give birth at such an old age. Sarah, however, was speaking about Avraham's age, not about herself. True, Chazal say that G-d changed Sarah’s words for the sake of peace, i.e. so that Avraham would not be offended, but still – how could G-d utter a complete falsehood?
3) Sarah responded by denying that she laughed. How could Sarah make such a denial when G-d obviously knew the truth?
Here’s the approach of the Netziv:
At the end of last week’s parsha Avraham was given a prophecy that he would have a son. Undoubtedly he shared the news with Sarah, and she accepted it -- there is no question that Sarah believed that a miracle could happen and that she and Avraham could conceive in old age. As the Netziv puts it, Hashem could make a rock give birth if he desired!
At the beginning of our parsha, when the angels came, Sarah suddenly resumed menstruation and returned physiologically to her youth. She was forced to re-interpret the prophecy given to Avraham. It did not mean that miraculously she would give birth in old age, but rather it meant she would have an unremarkable natural childbirth as a physiologically young woman.
There was one catch to her re-interpretation. “After I have aged, I returned to my youth,” Sarah said, “But my husband is still an old man!” If Yitzchak’s birth was to be natural event accomplished by a reset of Avraham and Sarah’s biological clocks, then Avraham should have exhibited signs of a return to youth as well. That had not happened, leaving Sarah with questions.
Hashem’s paraphrase of Sarah’s remark was a deliberate double entendre. In truth, Sarah’s words were a confident expression of belief, and G-d's rephrasing captured exactly what Sarah meant (G-d does not lie). “Will I give birth at such an advanced age!” was a rhetorical question – of course not, because I am now a young woman. However, those same words implied that Avraham’s lack of return to youth presented an obstacle. G-d’s rephrasing, for the sake of hamony between Avraham and Sarah, caused Avraham to miss that implication. Instead of hearing Sarah’s words as a rhetorical question, a boast of her natural ability to conceive in contrast to his own apparent lack of youth, Avraham understood her words as a real question – “Can I conceive at such an advanced age?”
Avraham was told to confront Sarah and in his mind G-d was being critical of Sarah for questioning her own ability to conceive. Sarah, however, never doubted that she could have a child, given her return to youth. She therefore completely denied the charge made by Avraham. It was not her own ability to conceive, but rather Avraham's which troubled her. Since G-d had left Avraham in the dark as to her true meaning, Sarah chose to not elaborate further and perhaps hurt Avraham. A simple denial was all she made, and she left it at that.
Let me end off with one other observation the Netziv makes, and this is probably the most important take-away for us. In the pasuk (18:15) that tells us of Sarah’s denial and Avraham's insistence that she had in fact laughed, there are two “psik”s. A psik, Rashi later (18:21) explains, signals a pause. Sarah was faced with her husband, in G-d’s name, putting words in her mouth. Avraham was faced with his wife denying what G-d had told him to be true. A marital thunderstorm was brewing! Sarah could have defended her words and revealed her true meaning at the expense of making Avraham feel inadequate. Avraham could certainly have lashed out at Sarah for her denial. But that’s not how either one responded. Before either one spoke, the Torah sticks in a psik – a pause. Time to take a breath and think. Time to consider the impact before speaking. Avraham and Sarah each tried to soften the blow and defuse the situation with calm.