What causes coins to turn red when they react with the air?

Coins turn red when they react with the air because iron inside them meets oxygen, and they become rust.

Imagine your favorite toy car, it’s shiny and new. But if you leave it outside in the rain for a long time, it starts to look rusty and not as bright anymore. That's what happens to coins too!

Coins are usually made of iron, which is a strong metal found inside many objects like nails or your bike frame. When iron meets oxygen from the air, especially if there’s a little water around, it starts to change color, turning reddish-brown, just like rust on a nail.

This reaction doesn’t happen all at once. Think of it like a slow dance between iron and oxygen. Every day, a tiny bit more of the coin changes color until you notice it’s looking redder than before, like your toy car after a long rainstorm!