Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
Out of your pennies, try to find the dirtiest one.
Take a small piece of paper towel and dampen it slightly with water.
With as much effort as you can, try for about 30 seconds to polish the penny with the paper towel. You'll probably find that not much is happening. Why do you think this is?
Now, take the pencil's eraser and try to "erase" the patina covering the penny. This time, if you put a bit of effort into it, you might see your penny slowly starting to become shinier.