Pennies build up dirt and rust over time. Soaking them in Coca-Cola can clean up the coins. The acidic nature of the soda eats away the dirt and rust, leaving a shiny penny. Don't leave pennies in Coca-Cola for too long, however, or the soda could damage the coins.
Pour Coca-Cola into a cup that you will not use for drinking in the future. The substances you are removing from the pennies may stick to the side of the cup and you don't want to ingest them.
Drop a penny or two into the Coca-Cola. The pennies should be completely covered by liquid and no pennies should rest on top of each other. Overlapping pennies can hinder the process.
Soak pennies for about 24 hours. Remove from the Coca-Cola and rinse with water. Lay pennies on towel, pat dry and leave coins to air dry. If pennies aren't clean, repeat the process.
Don't allow the pennies to soak more than 24 hours at a time. It might dissolve completely. You can always put a penny back in the Coca-Cola.