Okay, this sounds dumb. But hear me out. Drop two or three ice cubes into your stained mug.
Add a tablespoon of salt right on top of the ice. Swirl it around until the ice melts a little.
The salt acts like a scrub, and the cold water loosens the stain. It’s like a mini spa treatment for your cup. Your mug will feel refreshed.
Rinse and admire the results. You’re a genius now.