NUM159 Separate challah from dough. Man cannot survive without food, and for the majority of the world the staple food is bread. Accordingly, Hashem gives us a mitzvah to perform with bread dough, so that the bread is blessed and our souls gain merit. He gives us the opportunity to fulfill this mitzvah every time that we bake [in sufficient quantity], and through the mitzvah, our bread is nourishment for both the body and the soul. An additional reason for the mitzvah: so that the Kohanim, who are constantly occupied in serving Hashem performing the avodah in the Beis HaMikdash, shall live and have nourishment without effort, from our gifts of dough to them.