Why are my Pandora charms turning brown?

I'm curious, why is it that my Pandora charms are turning brown? Is this a common issue, or is there something specific I'm doing that's causing it? Are there any steps I can take to prevent this from happening in the future? I've taken good care of my jewelry and have kept it away from chemicals and moisture, so I'm a bit confused about why this is happening. Could it be the type of metal used in the charms, or perhaps the way they're plated? I'd appreciate any insights you can provide to help me keep my Pandora charms looking their best.