Today’s invention is a sharpening steel for penknives.
As you put your main knife blade away, you engage a boss on the fold-out steel with a slot in the blade itself.
On opening the blade next time, the blade drags the steel along the length of its cutting edge and then releases the boss.
(The steel would need to have its sharpening facets manufactured so that their direction varied along the length and were thus always at the optimal angle to the blade’s edge).
