Cornstarch, sugar, water, food coloring, medium saucepan, mixing spoon, sandwich-size ziploc bags
Stir together 1 cup cornstarch, 4 cups water, and 1/3 cup sugar in a medium saucepan. Heat on the stove until thick. Remove from heat and wait for it to cool. Pour the mixture into sandwich-size ziploc bags, filling each one halfway. Scatter drops of different food coloring into each bag, and have the kids squish the bags to turn the mixture different colors.