Why did Sarah laugh when she heard that she'll have a child despite her age (18:12) when she had just became a Nidah (Rashi 18:8) after not being a Nidah for a long time (18:11)? Shouldn't she realise that she can have children if she's a Nidah?
(Da'as Z'kenim Mi'Ba'alei Tosafos says that 18:11 means that she wasn't a Nidah for a long time, but now she was. Otherwise it'd contradict Rashi.)
Rav Hirsch points out that this was not a matter of disbelief in Hashem but the sudden shock caused her to laugh. Rav Hirsch implies that this laughter was from the same root cause as the laughter that overtook Avraham when Hashem first told him.
Rav David Tzvi Hoffman points out that Sarah had not been told the prophecy that Avraham had earlier been given, so it was a complete shock to her. At this point, they did not yet know that the guests were malachim and thought that they were normal travelers expressing the wish that Avraham and Sarah would receive a blessing.
הגם שאברהם כבר נתבשר על הולדת בן לשרה, הרי היא עצמה לא שמעה עדיין על כך, והנה שרה צוחקת, אמנם לא בקול, אבל לפחות בקרבה, בלבה, כי הרי לא ידעה, שבשורה זו מפי ה׳ יצאה.
She denied it in Vayera 8:15 because
She was afraid that she had offended the guest by laughing.
Indeed while Lech Lecha 17:17 is translated (like Onkelos) at Chabad.org as
And Abraham fell on his face and rejoiced, and he said to himself, "Will [a child] be born to one who is a hundred years old, and will Sarah, who is ninety years old, give birth?"
Rav Hirsch translates it as
Then Abraham fell on his face and laughed.
Rashi says that the expression of doubt, is because they found it hard to accept that they would merit such a great miracle. As Rashi says:
Will [a child] be born to on, etc.: There are questions which are positive assertions, like (I Sam. 2:27): הֲנִגְלֹה נִגְלֵיתִי, “Did I appear?” [meaning: “of course I appeared!”]; (II Sam. 15:27): הֲרֹאֶה אַתָּה, “Do you see?” [meaning: “of course you see!”] This too is a positive assertion, and so did he say to himself, “Was such kindness done to anyone else, that the Holy One, blessed be He, is doing for me?”
and will Sarah, who is ninety years old: Shall she be worthy of giving birth? Now although the first generations begot children at the age of five hundred, in Abraham’s time, the years were already lessened, and weakness had come to the world. Go out and learn this from the ten generations from Noah to Abraham, who hastened to beget children at the age of sixty and seventy.
Rav Hirsch translates this as:
Then Abraham fell on his face and ...
Rav Hirsch explains in detail why the laughter occurred.
In any case צחק is produced only by noticing something which strikes as ridiculous. But where in the whole world, is there any greater absurd contradiction than the expectation that a hundred year old man and his ninety year old wife, who have never had a child in the course of their long married life, now, practically at the end of their lives, should get a son! And then, to expect a great, world-conquering nation to arise, to place the hopes of the whole future of mankind on this late-born only child -- whose very existence was not to be expected, and in any case would be early orphaned! That surely is -- only taking the ordinary natural course of events into consideration -- an absolute absurdity, placing the very greatest expectations on the very smallest, almost non-existent beginning.
This contrast was so great that even an Abraham, with all his full confidence in Hashem -- as was shown by his falling on his face -- could not help laughing at the absurdity of himself and Sarah still getting a baby. And this laughter was -- and is -- so important that it is again brought into prominence with Sarah and was to be perpetuated for all future generations in the name of the promised child.
A few answers:
1). Maharsha (Chiddushei Agaddot): She did not yet actually have niddah - that was after the Beracha; however, Avraham was careful to not only have regular food as Tahor (pure), he also held of the opinion that a lady is impure מעת לעת therefore the angels told Avraham not to eat the bread because she would become a niddah later that day (perhaps this is part of the Bracha) and so Sarah didn't know she was פרס נדה.
2). Maharal (Gur Aryeh): That she stopped having regular cycles (ארח כנשים) the word ארח would mean a regular cycle, but rather like an elderly lady which can happen. And in fact this demonstrated something totally abnormal compared to young women.
3). Me: The Gemara Niddah 21b says א׳׳א פתיחת הקבר בלא דם. Ie: Perhaps Sarah felt her קבר open and even if there was no blood seen - she had held that she is Tamei. So she knew there is a chance or that she is for sure Tamei but still perhaps "impossible" for her to have a child since there was no actual blood - and maybe this is why it says here in Rashi that she does have ארח כנשים because her womb opened with blood - but later the Torah reports, she says she doesn't have ארח כנשים because she didn't realize there was indeed blood. And this is why we are using ארח here vs with רחל it mentions דרך (maybe).
(Baruch She'Kivanti) the Re'eim says something similar, that she felt her body return to having Niddah (ie: heavy limbs etc..) but not blood. So she was careful to keep the laws of Niddah but didn't think should could have a child.
4). Me: Perhaps Sarah had a slight misunderstanding in the Beracha: it says
ויאמר שוב אשוב אליך כָעֵת חיה...ושרה שמעת פתח האהל
Rough Translation: I will return to you like at this living moment... and Sarah heard from the opening of the tent.
Perhaps, Sarah misheard/misunderstood כָעֵת חיה and heard כְעֵת. The difference is that the first is she will have a child like the same moment (ie: next year at that time). However Sarah misheard with a Shevah, and it means, the amount of time it takes to have a child (Rashbam/Radak). If so she understood the Beracha is impossible (even for a young girl) since she just had a נדה. Avraham would separate for approx 2 weeks. The issue: a). A Beracha would be a perfect pregnancy and this would not be a complete pregnancy by definition (since it would need to be the time it takes to birth; however minus two weeks) and so she thought it must be incorrect. Similarly, perhaps she knew Yitzchak was connected to Pesach, and so either he would be conceived or born on Pesach and now both are impossible.
Support: It comments she was at or heard from the opening of the tent, ie: not directly, implying she misheard. And Hashem repeats the Beracha in verse 14 it states explicitly למועד אשוב, meaning that time of year (ie: Pesach) - ie: Hashem clarifies it is the same type of moment (ie: this was mistaken originally) and restates כָעֵת חיה.
Rav Yosef Yehudah, the son of the Sorotzkin Rebbe, gives I think a nice explanation to this question in his sefer Magged Yosef. Long story short, she wasn't surprised. She was faking it, as will be explained.
The verses in discussion are as follows (Genesis 18:10, 12, 13, 15):
ויאמר שוב אשוב אליך כעת חיה והנה-בן לשרה אשתך ושרה שמעת פתח האהל והוא אחריו: ותצחק שרה בקרבה לאמר אחרי בלתי לי עדנה ואדני זקן: ויאמר יקוק אל אברהם למה זה צחקה שרה לאמר האף אמנם אלד ואני זקן: ותכחש שרה לאמר לא צחקתי כי יראה ויאמר כי צחקת
[The Angel] said: “I will surely return at this time [next year] and behold, Sarah your wife will have a son”. Sarah [in the meantime] was listening at the entrance to the tent, and he/it was behind him. Sarah laughed within, saying: “After no longer having my period? As well, my husband is old?!” Hashem said to Avraham, why is it that Sarah laughed, saying: “Is it true that I’ll give birth, since I am old?” Sarah denied [this], and said, “I didn’t laugh!” because she was afraid. He said: “Actually, you laughed”.
To answer this question, some ambiguities of the story/the verses need be clarified. There’s a strange part in verse 10, which says:
ושרה שמעת פתח האהל והוא אחריו
It means that Sarah was listening at the entrance of the tent, which makes sense. The next two words however, are ambiguous. It says “he” was behind “him”. Who was behind who? Rashi explains1 that “he” is referring to the entrance (since there is no “it” in Hebrew, it refers to the entrance in masculine). He also explains “him” is the Angel that was speaking. Therefore, the verse is telling us that the Angel didn’t see Sarah listening behind him. According to this explanation, what does this detail add to the story? The Angel shouldn’t have minded Sarah listening, especially since he was talking about her. Why do we need to know this?
Targum "Yonasan"2 has an interesting translation of the last two words, not like Rashi. He writes:
וישמעאל קאי בתריה וצית מה דאמר מלאכא
Yishmael, Avraham’s son from Sarah’s maidservant Hagar, was listening as well behind the Angel. He was listening to what the Angel was saying. The Targum Yerushalmi2 says the same, based on the Midrash3. With this explanation, Sarah’s reaction can be understood. Sarah saw Yishmael listening, and was worried he would become upset at the news of her having a son. Until now, Yishmael had the status as Avraham’s first born. Hashem promised once Yitzchak is born, he’ll become the main son4. Previously5, Chazal taught us that Sarah cast ayin hara on Hagar, causing her to miscarry. Sarah was worried Yishmael would do the same to her, as revenge for her mother. This is why she laughed. She believed the news was true, but she hoped Yishmael wouldn’t take it seriously if she appeared to laugh at the Angel.
This fits very nicely with the strange words ותצחק שרה בקרבה, literally “she laughed with her innards”6. Rashi explains7 that she “looked” at her innards in disbelief and said: “how could this womb carry a child?” It sounds like she said this out loud. The reason would be because Yishmael was listening in. Chazal teach us8 that the Egyptian King Ptolemy ordered seventy Sages to translate the Torah into Greek (known today as the Septuagint). Chazal list different changes the Sages made in their translation9, since the literal translation would appear problematic to the uninitiated. Instead they translated it according to the proper interpretation. This verse is one of the changes. Instead of ותצחק בקרבה, they translated it as ותצחק בקרוביה, she laughed with her relatives. Meaning, Yishmael.
With that explanation for why she laughed, why was Hashem upset at her? He knew her intentions were proper. The reason is because according to the strict middas hadin, letter of the law, she showed a lack of bitachon, faith in Hashem. He promised her she would have a child; what was there to be afraid of? Someone of her stature should have had full faith that all would go as foretold. This is why she responded with לא צחקתי כי יראה, I didn’t laugh, rather I was afraid. She meant she believed she would have a child. At the same time, she was afraid of Yishmael. Hashem responded10 that she still laughed. Despite her faith, it was not complete.
1 To Genesis 18:12
2 Ad. loc.
3 Bereishis Rabbah 48:16. This explanation is also brought by the Chizkuni ad. loc.
4 Genesis 17:19
5 Ibid 16:6
6 Ibid 18:12
7 Ad. loc.
8 Megillah 9a
9 It was a miracle that they all separately made the same changes
10 Cf. Targum Yonasan, Ramban, Ohr HaChaim ad. loc.
The Kli Yakar says she never doubted Hashem's promise, but she was worried about Avraham living long enough to raise him. She wasn't concerned about her own age, because her sudden rejuvenation demonstrated that she had merited a miraculous intervention, and the same merit would surely grant her long years. But she didn't perceive any rejuvenation in Avraham, so she assumed his merit had been depleted in his battle with the Four Kings.
So when she said אחרי בלתי היתה לי עדנה, it wasn't a challenge. She was noting the fact that her youthfulness had returned. The only problem was ואדני זקן, Avraham was still elderly and wouldn't merit to raise his son. When Hashem recounted her words to Avraham, "He altered them for the sake of peace" and reported that she had doubted her own merit instead of his: האף אמנם אלד ואני זקנתי, "even if I have a child, I am old [and will not merit to raise him]." Hashem's response was היפלא מה' דבר, assuring them that they would both live many more years.
The rishonim I've seen say she thought it was a random incident of bleeding, as sometimes happens after menopause.
I once asked this question to Rav Belsky zt"l and he answered that she was an ailonis who wasn't physically capable of bearing children.
