In a small pot, mix together the baking soda and cornstarch. Stir in the water until completely blended.
Place pot over medium heat and cook, stirring constantly for 10 to 15 minutes until the clay begins to form. Remove from heat and continue stirring until the mixture becomes a sticky but soft dough.
Allow the dough to cool to the touch.
Place dough on a surface coated with additional cornstarch. Begin kneading the clay adding more cornstarch as needed until the clay becomes smooth and the tacky feeling is gone.
To store, wrap tightly in plastic wrap and place in an airtight container.
To dry, place shapes on a drying rack. If needed, flip shapes over after one side has dried. Depending on the thickness of the object, the clay may take up to 3 days to dry completely.