which cleaner will clean a cooper penny

	Project title: which cleaner will clean a cooper penny
Difficulty level:	Elementary school
Main link:	None

Hypothesis:

I think the vinegar will clean the penny best. MY other two cleaners I am using are salt and water, and vinegar and salt.
What is the stuff on the penny that makes it dark?

Materials:

3 dirty dark pennie.
3 cleaning solutions: vinegar,vinegar and salt, and salt and water.

The Procedure:

Put one penny in each solution.Watch. Note different changes on different days

Results:

so far vinegar is cleaning the penny.
Please help me find why the venigar is working best. What is the reaction?