A student puts some cooking oil and some water in a boiling tube and shakes it. She then examines it carefully.
The oil and water separate out, with the oil floating on top of the water.
Shaking it harder produces the same result.
Explain what this experiment shows.

The experiment shows that oil is less dense than water

Explanation:

If an object is less dense than water, it will float when placed in water, if an object is denser than water it will sink when placed in water.

The experiment shows that oil is less dense than water and so, the oil and water separate, with the oil floating on top of the water as a student puts some cooking oil and some water in a boiling tube and shakes it.