Equipment:
3 iron nails
3 glasses (small wine or sherry glasses are ideal)
cooled water (boil a kettle of water then allow the water to cool until it is just warm)
Olive or sunflower oil
Wire wool / fine sandpaper

Method:
Clean each of the nails with wire wool or fine sandpaper. Fill two of the glasses with cooled water, enough to cover the nail, but leaving a fingers depth unfilled. Leave the third glass dry. Add a nail to each of the glasses, and then pour a layer of oil over the water in one of the glasses. The oil will float to the top of the water and form a separate layer which should be about as thick as a pencil.
Carefully place the experiment somewhere that it can remain undisturbed and easily observed for 24 hours.
The experiment should look like the picture below :

Observe the mixture after a few hours; take notes of what you can see. Has any corrosion taken place? If you see corrosion, what is the colour of the corrosion that you see? Rate the level of corrosion of each of the items from 0 to 5 (0 being no corrosion, 5 being very corroded). Repeat this observation after 24 hours.