Coffee stain cleaner.

So, as with all coffee mugs, my one new-ish coffee mug shows coffee stains on the inside from the super-dark espresso that I drink vast quantities of each day, only it shows more on the inside than typical, since it's made of light ceramic.

And, it's interesting, using your thumb nail will scratch off a bit of the scummy stain, though if you use a scratch-safe dishwashing sponge, you have to scrub a lot harder than you think, since when you go to scrub it it actually mostly just seems to somehow form some kind of thick oil underneath it and move that thin tough sludge around on the inside, but never get it off.

Then, what happens to my surprise, but I'm craving milk, and I buy some organic milk the next time that I'm at the local hippie co-op, and later when I pour some dry oats and brown sugar into my coffee mug, I then put some 2% milk all on top of that, and then as I stir it then eat, I notice that the stain is gone, just completely gone from all inside of my coffee mug.

Something in the milk-fat must have gotten it off, without my having to really do anything!

I bet that has to do with chemistry, somehow.