Materials:
A plastic bottle with a lid (emptied)
Water
Food coloring
A dash of cooking oil
Detergent

Steps:
Pour the water into the bottle until half the bottle is full.
Add 1 to 2 drops of food coloring to the water.
Add a similar amount of oil to the water.
Screw on the bottle’s lid and shake the bottle for a few seconds.
Set the bottle on a surface and watch what happens. At first, it will seem like the water and oil are mixed together. Within a few minutes, however, and oil and water will slowly separate.

So, how does this happen? Why did the oil and water separate?

Even though water can easily mix with many liquids to create solutions, this is not always the case. This is because of the chemistry saying: “like dissolves like.” Water molecules are attracted to each other, and oil molecules are attracted to each other. However, water and oil molecules aren’t attracted to each other. When water encounters oil, it will stick with other water molecules rather than attach to oil molecules. Oil also has a lower density, or specific gravity, than compared water, so the oil will gradually separate from the water and remain on top of the water.

You can shake as hard as you can, but water and oil can never mix!