We all understand the meaning of this idiom, but I decided to look it up. What I found was a controversy over whether the stone in question was used by millers or knife grinders. Here's the link, which includes a photo of a grindstone:
In hopes of breaking up the monotony of winter weather, here are bird photos from a trip back in September. I didn't get to spend much time with my husband on his business trip to Florida, but I did work on my fiction, and go for walks. This post shows Florida birds I encountered on the well-groomed walking paths.