Heat oven to 350ºF. In a large bowl combine flour, baking soda, and salt. Cut in margarine until crumbly. Add water, stir just until mixed. Turn dough onto lightly floured surface, knead until thoroughly mixed, about 3 minutes. Roll out dough,  1/4 at a time, on well floured surface to 1/8 inch thickness. Cut into 2 inch squares, place 1 inch apart on ungreased cookie sheets. Prick two or three times with fork, sprinkle with salt. Bake for 20- 25 minutes or until lightly browned. Makes 8 dozen