Shake salt on top of the oil while you count slowly to 5. Add more salt to keep the action going for as long as you want. They did it over and over again and kept trying to touch the layers. Oil floats on water because a drop of oil is lighter than a drop of water the same size. Another way of saying this is to say that 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 in water. Things that are more dense than water will sink. Even though oil and water are both liquids, they are what chemists call immiscible liquids. That's a fancy word that means 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.