Monday tip of the week - getting stains off of mugs

I have a beautiful mug that I keep at work for water, but during the winter it also doubles as a tea cup.  I wash it by hand throughout the week, but it gets very heavy use.  Upon returning to my office one Monday, I noticed how dingy it was looking.

I brought it home and ran it through the dishwasher, but it didn't remove the the rings or the "grunge".  I remembered how great our shower turned out after attacking it with a Mr. Clean Magic Eraser, and I thought "what do I have to lose?"
I used a new eraser that I had on hand, and within seconds the ring was gone!
Horrible picture, I'm sorry.  The clean part is to the right, with the ring still over to the left.