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The negatively charged oxygen atoms in our air are attracted to the positively charged copper atoms in the penny. When oxygen binds with copper, they form a new molecule known as copper oxide. This is why most pennies you see look dirty or tarnished, it's actually copper oxide. Silver will not oxidize as quickly, as it is not made of a material that negatively charged oxygen atoms are attracted to.

a motor homunculus represents a map of brain areas dedicated to motor processing for different anatomical divisions of the body.

homunculus represents the importance of various parts of your body as seen by your brain. as a result, they take up a lot of brain space. although the cortical homunculus is a curiosity, penfield's work in mapping the brain's relationship to the body was invaluable.

the answer to your question is tooth decay.