Rashi, Exodus 15:25
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שם שם לו: במרה נתן להם מקצת פרשיות של תורה שיתעסקו בהם, שבת ופרה אדומה ודינין: ושם נסהו: לעם, וראה קשי ערפן שלא נמלכו במשה בלשון יפה, בקש עלינו רחמים שיהיה לנו מים לשתות, אלא נתלוננו:
There He gave them: In Marah, He gave them some sections of the Torah so that they would busy themselves with them, namely [they were given the laws governing] the Sabbath, the red cow, and laws of jurisprudence. — [from Mechilta and Sanh. 56b] and there He tested them [He tested] the people and saw how stiff-necked they were, that they did not consult Moses with respectful language, “Entreat [God to have] mercy upon us that we should have water to drink,” but they complained. — [from Mechilta] [translation by Judaica Press]
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Suggested Discussion Questions:

Why does it matter to "ask nicely" rather than to complain?

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Time Period: Medieval (Geonim through the 16th Century)