We tested 4 different methods as outlined on e.how.
1. Ketchup 2. Baking soda and water paste with toothbrush 3. Lemon juice and salt [or vinegar and salt] 4. An eraser
We tried a penny with each method and then tried different combinations. The lemon juice and ketchup both had the most dramatic results. And it was fun to put a penny in the lemon juice right after the baking soda for extra bubbly results.