Sarah's internal laughter
The qusestion was brought up in a number of places on Divrei Chaim, including:
Now we'll take a different perspective that my grandfather presents. His interepretations of Vatisachek Sarah bekirba [Sarah laughed within herself] signifed that Sarah was herself unaware of the laughter that arose within. It was not just inaudible but unconscious.
It was only in the depths of her soul that there arose some glimmer of doubt about the truth of what the angels said because acharey baloti hayta li adena [after I've grown old/worn I've become rejuvenated]. In this interpretation, leymor [saying] does not signify a quote but the reason. Sarah would not have said say those words at all. which is why it doesn't say vatomar. The sentence is the reasoning behind a laugh rather than one that was actually spoken. Sarah didn't even feel the incredulous laugh that only registered deep inside her.
Accordingly, my grandfather finds the Targum Unkuloos appealing, for it translates Hayipaleh meHashem davar not as "Is anything too wonderful for G-d," but as "Is anything hidden from G-d?" Even the subconscious laugh that was hidden to Sarah herself was revealed to Hashem.
He cites the Or Hachaim and Kli Yakar who follow a similar direction about the declaration of nothing being hidden from Hashem, though they do not insist that Sarah was unaware of the laughter in the same way. The Kli Yakar's take is that her fear prevented her from letting the laughter out, though she still knew that she felt it.
Now for another point
Why was Lot's wife turned into a pillar of salt?
There are some answers associated with actual salt that she asked her neighbors for to bring their attention to the guests. My grandfather has another take, though. He says that salt can only be eaten along with something else. It doesn't qualify as food on its own. In the same way, Lot's wife had no merit on her own and was only able to be saved when she was linked with her husband. When she put herself on her own, so to speak, by daring to look back, she lost that protection and turned into what she essentially was.
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