Midrash sur Le Deutéronome 4:20

“If the iron is dull, and one did not whet the edge, he must intensify his exertion; the advantage is in preparation with wisdom” (Ecclesiastes 10:10).
“If the iron is dull,” if the teacher’s face is dull toward his student like iron,⁵⁰The teacher is not interested in expending energy to explain the material to his students. “and one did not [whet] the edge [panim],” and the teacher does not show the student a friendly countenance [panim], “whet [kilkal],” there is a deterioration [kilkul] in the actions of the student. “He must intensify his exertion” – what should [the student] do? He should bring ten people who will appease the teacher and, ultimately, he will have much preparation for wisdom.⁵¹He will be in a much better position to attain wisdom.
Another matter: “If the iron is dull,” if the student’s face is dull toward his teacher, as it is stated: “Iron sharpens iron; [so a man sharpens the countenance of his friend]” (Proverbs 27:17),⁵²The teacher could be sharpened by the questions and insights of his student, but this student is dull and not insightful. and [therefore,] the teacher does not show the student a friendly countenance, “kilkal,” there is a deterioration [kilkul] in the actions of the student. “He must intensify his exertion” – [the student] should bring ten people who will appease his teacher. “The advantage [veyitron] is in preparation [hakhsheir] with wisdom” – ultimately, he will relinquish [motir] his anger and will enable [veyakhshir] him [to succeed in] his studies.
Another matter: “If the iron is dull,” if your studies have become dull like iron, “and one did not whet the edge,” [your teacher] cannot come to explain it to you, overcome it with your own ability.⁵³If your teacher is no longer alive and you have unresolved questions, work hard to figure out the answers on your own (Etz Yosef).
One of the students of Rabbi Shimon ben Yoḥai forgot his studies. He went crying to the cemetery. As he was crying profusely, he was shown a dream, and [a voice] said to him: When you cast three pebbles at me I will come. That student went to an interpreter of dreams and recounted the incident to him. [The interpreter of dreams] said to him: ‘Recite your studies three times and it will come to you.’⁵⁴Recite each thing you learn three times and you will remember it (Matnot Kehuna). He did so, and so it was for him.
Rabbi Berekhya said: If the nation whose power is as strong as iron, as it is written: “And the Lord took you and brought you out from the iron crucible, from Egypt” (Deuteronomy 4:20), becomes dull; “and one did not whet the edge” – and the Holy One blessed be He is not forthcoming toward the generation; “kilkal” – there are evil deeds in this generation. What shall they do? “He must intensify his exertion” – let them decree fasts, and the Holy One blessed be He will grant them eternal forgiveness.
Rabbi Ḥama bar Pappa said in the name of Rabbi Yehuda ben Rabbi Simon: If the skies above your heads have become dull like iron,⁵⁵There is no rain. as it is stated: “I will render your skies like iron” (Leviticus 26:19), and the Holy One blessed be He is not forthcoming, there are corrupt [kilkal] actions in the generation. What shall they do? They shall decree a fast, and the Holy One blessed be He will provide relief to His world.

Rabbi 'Aḳiba said: The executioners of Pharaoh used to strangle the Israelites in the walls of the houses, || and the Holy One, blessed be He, heard their cry, as it is said, "And God heard their groaning, and God remembered his covenant with Abraham, with Isaac, and with Jacob" (Ex. 2:24). Further, they burnt their children in the furnace of fire, as it is said, "But the Lord hath taken you, and brought you forth out of the iron furnace, out of Egypt" (Deut. 4:20).