Into your mixer, plase the yeast, sugar, salt, eggs, margarine and 2 cups of the flour. Start to mix, gradually adding the flour until the mixer will no longer go. You may want to change to the dough hook and continue to add flour until it is all used up. Transfer to a well floured board and start to knead the dough. Add additional flour as needed until the dough has a "silky" feel. This is when the dough no longer sticks to your hands when you hold it, and should take about 10 minutes of kneading. Place the dough in a well oiled bowl and let rise until double. (You will know it is ready to punch down when you put two fingers into the dough and it does not come back to you) Punch dough down and let rise again until double. After the second rising, place dough on board and cut into about three equal pieces. (If you want smaller challas, cut four.) Braid as desired and place on a cookie sheet which has been sprinkled with coarse corn meal. Using that reserved egg yolk (spill off the water you covered it with to keep it), add 2 teaspoons of water and beat until mixed. "Frost" the challas and sprinkle with either poppy or sesame seeds. Allow to rise until about double. Preheat oven to about 425 degrees and bake the challas for about l5 minutes, reduce oven to 350 degrees and finish baking, ( Challah will sound hollow when tapped with your fingernail ). This should take about another 20 - 25 minutes. Remove from cookie sheets to racks to prevent the challah from "sweating". They freeze well and can be made with raisins for the New Year, I just eliminate the seeds. Posted to JEWISH-FOOD digest Volume 98 #022 by "Ruth Frier" <antruthy@flash.net> on Jan 11, 1998