The Rogotchover: Benching
In the prophet Yeshayahu’s descriptions of the age of Moshiach (Chapter 11), we find the passage, “And the wolf shall dwell with the lamb, and the leopard … with the kid, and the calf with the young lion … and a little child shall lead them.” These last words are often taken to reflect the state of utter docility which will prevail during the Messianic Era. Yet, there are no mere metaphors in the Torah. In truth, only “a little child” would be allowed to lead these combinations of animals, for their joining constitutes the prohibition of k’layim (forbidden combinations), and thus not even a child of educable age would be allowed to lead them, but only “a little child.”
Why (in Bircas Harnazon) do we say Magdil during the week and Migdol on Shabbos and Yom Tov? The Gemara (Shabbos 116b) tells us that on Shabbos from Minchah onward we must not read from Kesuvim (Holy Writings), but we may read from Nevi’im (Prophets). Therefore, on Shabbos we make sure to say Migdol, which is quoted from Prophets (II Shmuel 22:51), maintaining a distinction from the weekday version, which comes from Holy Writings (Tehillim 18:51), which may be recited any time during the week.