In saucepan, stir together baking soda and cornstarch. Add the water and mix well over low heat, stirring occasionally, until it has a mash potato consistency(about 7-10 minutes). However, the last few minutes you will need to stir constantly as it thickens. Remove pan from heat and let cool. When cooled, turn it out on to a cornstarched board and knead dough witrh hands to make a workable clay. Separate into about 4-6 balls of clay. Take one ball of clay and make a small ditch or bowl. Then add a few drops of food coloring and fold over pinching sides(try to take care that food coloring does not seep out). Continue you this with the rest of the balls of clay using different colors of food coloring. As the kids mash it with their hands(improving fine motor skills) it will change into a surprise color. Allow creations to air dry overnight. Refrigerate any leftover dough in a ziplock bag for up to 3 days.