Just dispelling the myth that the bottom of a ceramic coffee cup will sharpen a dull knife.

What it WILL do, is help touch up and maintain the edge of an already sharp knife, but you can rub a dull knife on the bottom of a coffee cup until your arms fall off... you won't end up with a razor sharp knife.

Blades are sharpened in stages, starting with coarse material to set the edge and using progressively finer abrasives, until you have polished out all of the scratches and have a mirror finish. From there, you can maintain a sharp edge by using a fine abrasive such as etched steel, diamond impregnated steel or ceramic stones... or, if you don't have any of those handy, the bottom of a coffee cup will do a great job too.