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Has your kid ever looked at a collection of change in a jar or on the kitchen counter and asked why some coins are darker than others? Chances are you replied that some coins are older than others. The scientific truth is that copper atoms in the penny are attracted to oxygen atoms in the air which interact and form copper oxide. Some pennies look dull and dirty because they are covered with this infamous copper oxide. Our cleaning copper coins experiment is a great way to show your kid how to get those dingy pennies looking nice and new again.