Playing with clay dough is a kind of staple of childhood pass times. Use this simple recipe which dries and hardens without baking to fill up a long rainy day.
Put cornstarch and baking soda into a large saucepan. Stir to mix it up.
Stir constantly while adding a little water at a time. It will be very smooth and thin to start.
When it thickens to the point where you can't stir it, it's done. Put the dough on a cookie sheet. Cover it with a damp towel so that it doesn't dry out.
After it cools down, knead the clay until it feels nice and smooth.