The kids poured 1 cup of vinegar into the pot and squeezed lime drops, and then the pennies. The mixture of the coins with the solution began to make little bubbles.
We left the coins in the solution for approximately 1 hour and then we rinsed. Some coins were really shiny however some of the dirtiest ones didn't clean much, just superficially.
This was one of the easiest science experiments we did. The kids liked it a lot.