1 cup cornstarch
1/3 cup sugar
4 cups water
Mix cornstarch, sugar and water together in a saucepan. Bring to a boil stirring constantly. The mixture should look a little like Vaseline. Allow to cool.
Fill bags one-third full with the cooled mixture. Add food coloring (red, yellow, green, blue) to product in the baggie. Zip up and squish the bag. Soon a rainbow full of colors will appear.
Caution: Don't hit the bags hard, as the bags will open up or holes will be made in the bag.