1 c. cornstarch
2 c. baking soda

Mix these 2 ingredients together in a medium saucepan. Add:

1 1/2 c. cold water

Stir until smooth. Cook over medium heat, stirring constantly, until the mixture reaches the consistency of slightly dry mashed potatoes. The mixture will come to a boil then start to thicken, first in lumps and then in a thick mass.

Turn the mixture out onto a plate and cover it with a damp cloth while it cools. When it's cool enough to handle, knead it thoroughly on a cornstarch dusted surface until its smooth and pliable. Then store it in a tightly sealed container until its time to use.