Now I can show my kids the same cool trick. We started out with a handful of dirty pennies and a bowl of vinegar and salt. We placed our pennies one by one in the vinegar solution and then rubbed the grime off with q-tips.
So what's happening? After a while the copper in the pennies begins to react to the air to form copper oxide. When pennies are new the copper is bright and shiny, but the copper oxide changes them. It gives them a dull patina. The acid from the vinegar dissolves the copper oxide and makes them shiny and new again!
Here you can see where the vinegar has started to clean off the copper oxide. The brown parts are where the vinegar hasn't touched yet.
Once all the pennies were clean we took them out of the vinegar and rinsed them off. Then we had bowls full of vinegar with copper oxide floating around in it. We rummaged around in the garage until we found these nails. We soaked the heads of the nails in our vinegar and after about 10 minutes we had copper plated nails!
Happy President's Day!