Why did the workers who built the Mishkan not erect it?
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Rashi: Because, due to the weight of the planks, they were unable to erect it.
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And, because Moshe did not have a hand in Meleches ha'Mishkan, Hashem left it for him to erect. Even then, he only (pretended to) put it up but the planks erected themselves.
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According to the Gemara in Shabbos, 92b, whereas the people were unable to erect the Mishkan due to its size; Moshe was able to, since he was ten Amos tall (Sifsei Chachamim).
-And this explains why the Torah writes later (in Pasuk 17) "Hukam ha'Mishkan" the Mishkan was erected).
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Why did a number of Levi'im not erect the Mishkan together, just like they did whenever they traveled?
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Oznayim la'Torah: Because the initial erection corresponded to the creation of the world, before the angels were created, when Hashem set up Heaven and earth single-handedly.
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It now required someone of the caliber of Moshe Rabeinu, to put up the Mishkan singlehandedly like Hashem created the world.
What is the definition of "the Mishkan", "the Ohel" and "Kol Keilav", respectively?
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Ramban and Seforno: "The Mishkan" refers to the bottom set of curtains
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(consisting of threads of gold, Techeiles, purple and crimson wool, and twined linen) "the Ohel", to the curtains of goats-hair
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(and "Kol Keilav", to the Mishkan's accessories - Seforno).
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