What Causes the ‘Rust’ on Coins?

Coins can turn red because they react with the air. When coins are made, especially from copper or other metals, they have a shiny color. But when they are left out in the open, like in your pocket or on your table, the oxygen in the air starts to change their color. It's kind of like how an apple turns brown when it’s cut and exposed to air. The more time the coins spend with air, the redder they get.