sharpening a knife on a rock

Ran across this https://www.youtube.com/watch?v=cAfe8ZYnlSU Interesting this guy has a lot videos hawking some knife sharpening gadget but did this one off video of what to do if you lose the miracle gadget you bought from him. Gained lot respect in my eyes posting this. Reminded me of the Burrfection channel video of sharpening knife on a brick.

Anyway walking up driveway from mailbox today, I picked up couple flattish rocks. All rocks here are sandstone and many sort of flattish but none smooth and truly flat. Anyway I tried sharpening couple cheap little stainless paring knives on them. Surprisingly I got them improved from blunt to able to cut. I suspect work better with river rock smoothed by water. Still its interesting and in emergency, a sharper knife beats a duller one every time.