You need:

½ cup cornstarch
2 cups cold water

Food coloring or tempera paint

Then:

Mix cornstarch and water in a saucepan, stirring constantly to eliminate lumps. Bring to boil. Mixture will begin to thicken. Pour into several small containers (plastic laundry scoops, old muffin tins, and old film or yogurt containers). In each container, add a different color. Allow to cool before using. - Vicky Whitehead