Fun and very entertaining for the young ones!
Fill the tall glass/vase almost to the top with room-temperature water. Warm water is ok, too. Then pour a little oil into the other glass.
Add a couple of drops of food coloring to the oil. Use one drop of each color (red, yellow, blue and green), but you can use any color combo you wish. Don't be tempted to add lots of food coloring, your water will turn dark quickly.
-Tall glass or vase
-A smaller cup/glass
Less than 1 hour