Combine the flour, salt and baking soda. Add oil until it is thoroughly mixed in to form small crumbs. Add water two tbs. at a time, mixing well after each addition. Knead the dough on a lightly floured board for 1-2 minutes. Divide the dough in half and roll each half out on an oiled baking sheet with an oiled rolling pin to about 10" by 14" rectangle. The dough should be very thin, about 1/16" to 1/18" thick. Cut the dough in 2" squares and prick each square 3 times with a fork. Sprinkle with additional salt if desired. Bake at 350 degrees 10-15 minutes. Turn over for 5 minutes more. Makes 2-3 dozen wafers.