We used this is a science enrichment project during our money unit. The kids had a lot of fun and the sorting sheet helped keep the organized and on track.
I preceded the experiment with a presentation from the U.S. Mint site on how coins are made. We talked about the materials used for making pennies and the process of oxidation. I attached a "rainbow" of pennies to an index card with glue dots, and passed it around the room. It included a shiny new coin, one blackened and covered in verdigris, and everything in between. The children marveled at the breadth of shades. I had picked out a variety of brown, bronze, gold, copper, green, black, etc. crayons for them ahead of time and let them go to town, capturing the mottled colors of each penny as closely as they could. This was a simple, fun introduction to the scientific method. They enjoyed recording their "data" and hypothesizing about which substances would work the best.