Mix cornstarch and water together (start with 2 parts corn starch to 1 part water, i.e. 2 cups cornstarch with 1 cup water), add food coloring while mixing the cornstarch mixture. You may need to add a little more water but only add a small amount at a time so you do not want to water the mixture down too much. Now let you kids play with the goo. The goo has properties of both a liquid and a solid.