. Turns out, it's a new one for a lot of people—novel enough that we felt it was worth documenting here, complete with the full video above.I don't remember where I first saw this in action, but I do know I've been doing it since at least high school. Most ceramic plates, bowls, and cups have an unglazed rim around the very bottom, where the piece was in contact with the floor of the kiln. This unglazed ceramic is harder than metal and can be used just like a sharpening stone.
All you've got to do is flip the mug over and run the knife blade along it, holding it so that the edge maintains an angle of about 15 to 20 degrees. For the best edge, I like to start with 10 to 15 strokes on each side, then do five to 10 on each side, then four, then three, then two, ending with a series of single strokes on each side. This method creates a new edge that is relatively symmetrical, and it's remarkably effective.
In those rare cases in which you have a dull knife and cooking to do, but no ceramic in sight, the top edge of a rolled-down car window will work just as well. Even a smooth stone or slab of concrete outside can work in a pinch.
Even better than a ceramic mug is a china plate. Porcelain provides a finer and stronger sharpening surface than ceramic.