When my brother brought his friend from Poland to visit, we didn't know exactly what to expect. He wanted a cup of coffee in the morning and then asked for salt. Do the Polish put salt in their coffee?
The mug was stained from coffee.
He put salt in the cup.
He added a bit of warm water to make a paste.
This paste was used to clean the mug.
He used his fingers to rub the paste on the stains until they came off.
He returned a sparkling white mug to us.
Try it. It works!