Fireworks in a Glass is a great experiment for the children AND you. Everyone usually has these ingredients in their pantry, so it is very simple to set up.
3/4 glass warm water
2 tablespoons oil (I used olive)
Extra glass for oil / colouring
Skewer or fork for stirring
In one glass, add the warm water. In another glass put the oil and 4 drops of food colouring (1 drop of each colour if you have 4 colours). Using the skewer or fork, stir the oil / food colouring. Once stirred, add the oil to the warm water and just watch – do not stir. Let the oil settle and the magic will happen!
The science behind it: Food colouring dissolves in water, but not in oil. When you stir the food coloring in the oil, you are breaking up the coloring droplets (though drops that come into contact with each other will merge… blue + red = purple). Oil is less dense than water, so the oil will float at the top of the glass. As the colored drops sink to the bottom of the oil, they mix with the water. The colour diffuses outwards as the heavier colored drop falls to the bottom. (taken from About.Com)