Stir cornstarch into water and the solid particles will stay suspended in the liquid without actually dissolving. The result is called a non-Newtonian fluid — that is, one that sometimes behaves like a liquid and sometimes like a solid, depending on how much pressure it's under (think quicksand).
Once you've made it, play with it: Give your child
various utensils for digging and pouring — a slotted spoon, a
sieve — and perhaps plastic animals to bury. Have her to put a spoonful of Oobleck in her palm and then squeeze and release it.
In a large bowl stir together 1 cup of cornstarch with the water.
The Oobleck should fall in a thick ribbon when you lift the spoon.
If the mixture seems too dry, add water, a few drops at a time; if the mixture seems too wet, add cornstarch, 1/4 teaspoon at a time.