Particle physicists use graphs like the one shown on this page to look at the results of their experiments. By putting mass readings on a histogram, they can see that the peaks show separate particles or different versions of the same particle.
So, what do pennies have to do with particle physics? Pennies have an inner stucture that you can discern without cutting them open, just like protons have quarks inside of them, which particle physicists can see from the results of scattering experiments.
Over the span of months, many of us probably will let unspent pennies pile up in a bowl, or on a shelf, or in several coat pockets. YOU might never think to measure the mass of each one, but WE have. However, you can go ahead and analyze the data. You'll find that the interior of pennies is a lot more visible on a histogram than when the penny is in the palm of your hand.
We let you fine-tune 150 pennies by the minting year. You can see that something happened to the masses of pennies several years ago.
Learn more about histograms with an interactive example from Interactivate at Shodor. Choose "My Data" and try copying this penny data (text or excel) into Shodor's histogram tool to see how the weights vary. Can you see the two peaks?