主题：【原创】颠个小菜--在伦敦吃馆子 -- 瘦形胖子

it's a honing tool that sets the edge of the knife straight. When the knife becomes dull, it has lost its edge, and it needs sharpening. If you have very good knife, professional sharpening is recommended. It's extremely hard to do it right by yourself.

This video might help

http://www.youtube.com/watch?v=8hKXQHGwzAw

Sorry for the English. I'm also really tired - too tired to spend twenty minutes to type this up in Chinese.