Materials Needed:

A glass jar or clear drinking glass
Vegetable oil
Salt
Water
Food coloring (if you want)

DANGER!
Don't forget to be careful with glass.

Step by Step

1.Pour about 3 inches of water into the jar.
 
2. Pour about 1/3 cup of vegetable oil into the jar. When everything settles, is the oil on top of the water or underneath it?
 
3. If you want, add one drop of food coloring to the jar. What happens? Is the drop in the oil or in the water? Does the color spread?

4. Shake salt on top of the oil while you count slowly to 5. Wow! What happens to the food coloring? What happens to the salt?

5. Add more salt to keep the action going for as long as you want.

Why does the oil float on the water?

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.

What happens when I pour salt on the oil?

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 top of the water.

This looks like a Lava Lite. How does a Lava Lite work?

Like your oil and water, the "lava" in a Lava Lite doesn't mix with the liquid that surrounds it. When it's cool, the "lava" is a little bit denser than the liquid surrounding it. When the "lava" rests on the bottom of the Lava Lite, the light bulb in the lamp warms it up. As it warms up, the "lava" expands a little. When it expands, the "lava" stays the same weight but it takes up more space-so it's less dense. When it's warm enough, the "lava" is less dense than the surrounding liquid, and so it rises up to the top to float. At the top of the lamp, it cools down, becomes more dense, and sinks once again. This cycle repeats over and over as the "lava" warms up and rises, then cools down and sinks.

Exploratorium Teacher-in-Residence Eric Muller created this activity while playing with his food in a Chinese restaurant.