In a saucepan, stir together the baking soda and cornstarch.  Add the water, mixing well.  Cook the mixture over low heat stirring occasionally, until it resembles mashed potatoes, about 7-10 minutes.  You will need to stir it almost constantly the last few minutes as it thickens.  Remove the pan afrom the heat and set it aside.  When the dough has cooled, turn it out onto a floured board.  Knead with your hands, adding more cornstarch as needed, to create a workable clay.  For tinted clay, knead in a few drops of food coloring.  Allow creations to air-dry overnight.  Refrigerate any leftover dough in a ziploc bag for up to 3 days.