[fluoridation]

That wouldn't be a puzzle, then. It would some kind of engineering challenge. A puzzle starts by knowing the answer and then putting some circuitous path between it and the player, that they have to figure out how to navigate. It's inherently wasteful to construct puzzles.

[amelius]

Unless the people solving the puzzles learn something valuable on the way.

Anyway, I don't agree that puzzles by definition have known answers, unless you want to nitpick and I just change my "puzzle" into "challenge".