A few months ago, I was making breakfast for my wife and daughter in a rental home in Zell am Moos, Austria. The kitchen was cute and fairly well equipped, but, like all vacation-home kitchens, it had one major problem: crummy, dull knives, and no sharpening stone in sight. I smashed my way through a couple slices of tomato before instinctively reaching up for a ceramic mug from the cupboard to sharpen the knife.

Then I thought to myself: Does everyone know this trick? So I shot a quick video on my phone and posted it on Twitter and Instagram. 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. Within 30 seconds, I transformed a knife that could barely hack through a potato into one that could effortlessly slice a tomato.

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.

Note that this technique actually sharpens your knife by removing material—it doesn't just hone your knife, as running it along a steel (or along the back side of another knife) would do. It also tends to remove quite a bit of material compared with a dedicated sharpening stone, so I'd recommend using this technique only with cheap knives or when you're really in a bind. Doing this repeatedly will quickly cause your knife blade to shrink. Choose your mug carefully, too: The technique does not damage ceramic (it leaves a dark residue of shaved metal that can easily be washed off), but I wouldn't use your finest china for it!

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