Mix 1/3 cup sugar, 1 cup cornstarch, and 4 cups of cold water. You CAN double the recipe. Heat the mixture (do the night before) until it begins to thicken, stirring constantly. Cool. Divide the mixture equally into 4 plastic containers. Add food coloring – one color to each container until the mixtures reach the color intensity desired. Add 2 heaping tablespoons of EACH color mixture to a quart-sized zipper lock bag. Seal the bag and tape it closed with duct tape. You can also label each bag with the name of the colors enclosed, if desired. Let the children knead the bags, mixing the colors. Extension: Putting different amounts of colored mixture in the bag varies the colors created.
Cut, or have the children cut, a large red circle. Cut a smaller orange circle and so on through the colors of the rainbow. Glue the circles together in descending order on top of one another. When you are through, cut the final circle in half. This will create two rainbows- one for a bulletin board and one to send home. The circles are easier to cut than each individual strip of the rainbow. These are colorful and brighten any room or classroom!