Mix the sand and cornstarch thoroughly in the top of the double boiler (being careful not to scratch the bottom of the pan). Pour in the boiling water and mix well. Cook in the double boiler briefly until the mixture has thickened. Should it be too thick, add a little more boiling water. Let the clay cool for a bit, and you're ready to model!
When your piece is finished, put it on a flat pan or cookie sheet in a 275 degree oven until dry. You can dry it without the oven, but it will take longer.
Hint: Try mixing in some food coloring for variety!
(Adapted from Nature With Children of All Ages, by Edith A. Sisson.)