DMs: What's a bit of world building on the micro-scale you're proud of?

[Cozzer]

After a series of misunderstandings that would be difficult to translate in English (due to the fact that "bull", as in "Bull's Strength", is "toro" in Italian, as in a "toroidal shape"), the party's barbarian gave a random shopkeeper the idea of creating the "doughnuts of Bull's Strength", instead of the usual potions.

During the rest of the campaign, in the background, this random shopkeeper made it big and an ever-increasing percentage of magic potions was substituted by magic doughnuts, in the whole kingdom.