Place a copper penny in a bowl or on a plate outside. Check the penny each week to watch as the copper surface slowly corrodes from exposure to the elements. Corrosion will happen more rapidly if you live in a wet area or near the ocean.
Lay a penny in a bowl to experiment with rapid corrosion.
Pour 1/2 tsp. salt on it and then cover the surface of the penny with vinegar or lemon juice.
Wait five to 10 minutes and then remove the penny and place it on a paper towel or plate.
Watch the penny through the course of an hour change from bright and shiny -- the result of the acid in the vinegar or juice and salt breaking down and stripping away tarnish and dirt from the penny's surfaces -- to a green color as the copper reacts with the air.