I sometimes include puzzles in my games. The evil tree from the last game session is a puzzle monster, for sure. It cannot be defeated by brute force. I made that terribly obvious, to avoid the thousands of points of wasted damage seen with another puzzle monster, the hydra. No, you're not wearing it down. No, you're not bringing it ever closer to -10xHP. You're doing nothing at all.
So, obviously a puzzle.
What's the solution?
How complex is the puzzle?
From the GM's side of the screen, a puzzle can look very simple. But from the player's side, it can see terribly complex. If it seems complex from the GM's side of the screen, odds are it's going to seem (and be) unsolvable by the PCs. They're dealing with a limited set of information at best, different understandings of the words your using, a different perception of the situation, and your puzzle.
In C2 Ghost Tower of Inverness, The simple chessboard puzzle in the dungeon was also a big complexity for them, even though - once they got started - the solution wasn't complicated. But it was easy to see it as more than it presented. It was all too easy to see more issues where fewer actually existed. The rotating statues puzzle was simple at heart, but still required a lot of clues to solve it - and a lot of freedom of movement about the dungeon.
Is it even a puzzle?
Still, it's easy for the players to take bits of one puzzle or riddle and assume it's connected to another. You also get non-puzzles that are seen as puzzles - perhaps our Gamma Terra GM andi will tell the tale of the eight statues and the eight vials of holy water in the comments. One from our games was the beaded curtain in C2 Ghost Tower of Inverness, where the PCs spent an extremely long time dealing with something that was just a simple obstacle - to be either forced or removed via Dispel Magic.
Knowing is Half the Puzzle
With those two things in mind, I know I need to keep solutions simple and relatively obvious - or, at least, hinted at three or more times. The more, the better. I'm likely to keep it a little vague, maybe too vague, but throw a lot of them out. Even armed with all of them, the players won't always figure it out. That's okay, but without them I'm assuring myself that they can't. That's not fun for anyone.