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You will need:

A large drinking glass,
Water,
Vegetable oil,
Abuot 4 tablespoons of salt.

Half fill a glass with water

Add about half an inch of oil, such as cooking oil or vegetable oil, and wait for the two to separate.

What do you think will happen if you add a pinch of salt? Nothing? What about more salt?

Try it and see!

Explanation...