What makes pennies dull?

Answer 1

Most of the countries that issue coins called pennies or cents mint them with an outer layer of copper or bronze (*). Copper oxidizes (i.e. tarnishes) very easily because it reacts with everything from oxygen in the air to dirt and oils on peoples' fingers, so the coins quickly change color in circulation.

(*) Cents and pennies used to be made out of solid copper or bronze, but the cost of copper has gone up so much that nearly every country has changed the coins to be made of a cheaper metal like steel or zinc, with only a thin copper plating so they look the same as they did in the past.