One would be what the penny is made of; before 182, I think, the penny was heavier and another is age; over time the metal reacts with oxygen forming rust that can make it weigh heavier.. I am still trying to find out a third. I would think that maybe if the penny is chipped or even a train flattened one, maybe it would change the mass, but I am not sure about that though I am trying to find out if it does. Go to www.usmint.gov then click on Historians Corner ; History of the mint and then coin production.
