Musar su Esodo 12:39

... And on this the Torah says (Exodus 13:7) "there will not be seen and there will not be found" - it will not be seen b'maaseh (in action) and will not be found b'machshava (in thought), rather one should anul it in his heart. The mitzvot have 3 categories: mitzvot of speech, of the heart and of action, as it is written (Deuteronomy 30:14) "in your mouth and in your heart to do it". Comes the Torah (instructing us) to anul it in the heart, corresponding to the mitzvot which are dependent on the heart. Comes the 'kabbalah' (instructing us) to eradicate it from the house or to burn it, corresponding to the mitzvot of action. And to say 'kol chamira', corresponding to mitzvot of speech. In this way the 3 categories of mitzvot are fulfilled through the prohibition of chametz, teaching you that the prohibition of chametz incorporates all the mitzvot... Just as the 'kabbalah' comes (instructing us) to eradicate chametz and (livdok) to check the house in nooks and in cracks, so too we are obligated to search and check the chambers of our inner being for bad (machshavas) intentions and bad (hirhurim) thoughts. Just as bedikat chametz (checking for chametz) is not valid by sunlight, nor by moonlight, nor by the light of a torch, but only by the light of a candle, so too the bedikah (checking) of the yetzer hara must be by the light of the neshama (soul) which is called 'ner' (candle), this is what is written (Proverbs 20:27) "the candle of Hashem is the soul of man, which searches the chambers of one's inner being.

Our sages in Baba Metzia 86, explain that the reason the bread was never brought to the table was that Sarah had unexpectedly menstruated, become ritually unclean through having become biologically rejuvenated as of that day. 18,11 which reports that Sarah had experienced her menopause some time ago, refers to her situation prior to the arrival of the angels. It was Abraham's custom not to eat anything that was not ritually pure. Still, even assuming that Abraham personally conducted himself on that basis, how could he apply such stringent yardsticks of conduct to others? One simple answer is that it is not good manners to offer guests dishes that the host himself is not prepared to partake of. Rabbeinu Bachyah answers the problem quite differently. Sarah had stopped handling the dough the moment she became aware of her condition, since she knew that otherwise her husband would not touch the bread. Abraham therefore had no need to worry about that. However, Abraham was concerned about the חמץ aspect, seeing the date of the angels' arrival coincided with the calendar date on which Passover would occur in the future. During the time in which Sarah had to make arrangements for someone else in her household to handle the dough, the dough could have become leavened and as such forbidden. The date is alluded to by Abraham saying to Sarah לושי ועשי עגות, "knead it and make into cakes." The expression עגות, occurs in connection with מצה, i.e. עגות מצות, Exodus 12,39. Since Rashi does not follow the above line, we prefer to accept the approach of the Midrash, who regards the whole procedure as sacrificial gifts. It is of course, totally inadmissible to offer as a sacrifice something that is ritually unclean, hence the bread could not be served.