You will need white vinegar, salt, some tarnished pennies and some shiny new screws, nuts and/or bolts. Pour 1/4 cup vinegar into the bowl and add 1 tea. salt. Stir. Put about 5 tarnished pennies into the bowl and count to 10 slowly.
You will see the tarnish disappear, revealing the pennies' copper color. Take out the pennies and rinse them out in some water. The acid in the vinegar reacts with the salt to remove the tarnish or copper oxide.
Now, add more pennies to the bowl for 10 seconds, but this time, don't rinse them off. Place them on a paper towel to dry off. In time the pennies will turn greenish-blue as a chemical called malachite forms on the pennies as the copper atoms from the pennies, oxygen from the air, and chlorine from the salt combine.
Now, just for fun, place some nuts and bolts in the vinegar and watch as they become copper and tarnished in color. The copper ions from the pennies attached to the nails because the positive copper ions are attracted to the negatively charged nuts and bolts.