First, assemble cleaning ingredients in small containers. Ryder recommends testing a few different ingredients, to see which work the best. Yesterday, he used these:
2. Baking soda and water
3. Vinegar and salt
4. Water and salt
Other ingredients you might want to try, alone or combined with one another or the above:
2. Dish soap
3. Lemon juice
4. Carbonated water
5. Water and sugar
6. Other beverages you have in the fridge; particularly ones your kid convinced you to buy because the packaging looks cool, but which taste disgusting and no one wants.
If you have an old toothbrush, you can use it to rub the ingredients on the pennies, which helps with the cleaning process. It's extra great if the 1 year old then comes in and grabs the old dirty ketchup and salt-covered toothbrush and starts cleaning her teeth.
To make this into an experiment, start with a bunch of pennies of the same level of dirtiness (it helps if they were all made either before or after 1982, when they switched from 95% copper to mostly zinc with copper plating [note that some 2009 pennies are mostly copper]). Try one solution on each penny, and then compare the results. Or, try one solution for different amounts of time on each penny. Take notes about your experimental setup and observations to make it official. Happy penny cleaning!