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.
The eraser was our least favourite.
