What you need:
A glass jar or clear drinking glass, vegetable oil, salt, water, food colouring
What you do:
Fill half the jar with water.
Pour in about 1/3 cup of vegetable oil into the jar. Allow your two layers to settle.
Add one drop of food colouring to the jar. Is the drop in the oil or in the water? Does the colour spread? Do not stir the colour in.
Shake salt on top of the oil while you slowly count to 5. Wow! What happens to the food colouring? What happens to the salt?
Add more salt to keep the action going for as long as you want.
How it works
Firstly, the oil floats on top of the water because water is denser than oil. Density is a measurement of how much a given volume of something weighs. Things that are less dense than water will float and things that are more dense than water will sink.
Even though oil and water are both liquids, they are called immiscible liquids. That's just a fancy scientific name meaning they don't mix.
Salt is heavier than water, so when you pour salt on the oil, it sinks to the bottom of the mixture, carrying a blob of oil with it. In the water, the salt starts to dissolve. As it dissolves, the salt releases the oil, which floats back up to the the top of the water.