Let me give you some examples.
First, here's a common "Just an answer" riddle one could use in a game:
You come to a dark river in the underground labyrinth. Inscribed on the shore is a message:
If I have it, I want to share it.
If I share it, I no longer have it.
What is it?
(Answer after the jump)
The answer is, of course, "A Secret."
But in rpg game terms, the only way to use the answer to that riddle is to use it as a password, or magic word. In fact, in a game I played recently, that exact riddle was used to create a magical bridge across said river. It was pretty awesome, because the game was super-fun. But it got me thinking that there's something about someone only saying the answer out loud and solving it that doesn't carry the weight versus requiring an action you carry out from figuring out the riddle.
Let me give you another example*:
You open the door and immediately see that the room is filled with swarms poisonous insects running along the floor and walls. They don't seem to cross the threshold to the hallway you are now standing in. At the far end is a closed wooden chest on a pedestal. Inside the door a message is inscribed:
Rust Demise and Fire's Mirth
Water Walker, Squeaking Dearth,
Wear me now upon your skin
And safely you may enter in.
Here we have a riddle that requires a player to put their money where their mouth is. Its not enough to yell "Oil is the answer!" To get the treasure, they have to then be willing to coat themselves in the substance and walk into the room. Only then then the insects avoid them will they know for sure that they solved the riddle.
I feel like that kind of riddle has more impact on a player (and character) than just saying a magic word, sometimes at random (Mellon... I'm looking at you).
Here's another riddle* I used in a recent one-shot "Shadows Over Eternia" game for another example:
This room contains a desk, some broken down bookcases, and an old fireplace. Over the fireplace hangs a rusty sword and axe. On the desk is an open diary, damp and moldy. Only the top page is legible:
Provider, devourer, a double-edged blade,
Man he has tamed her, accepting this trade.
Revenously hungry,must all the time feed,
Yet drink she must not, this can kill her indeed.
Once the players figured out the answer was Fire, they then had to decide what to do with that answer. It didn't take long for them to light a fire in the fireplace, which of course slid away revealing a secret compartment. In this case there wasn't any danger in them trying different things, but it made it so there was more to the riddle than just the answer, there is the question of what to do with the answer.
So I guess the point of this post is just that I think you consider using riddles in your rpg games, but when you do try to make the answer only part of the puzzle. I feel its a lot more memorable for the players if they have to coat themsleves in oil based on a hunch and brave the insects than it is to just yell a word and a treasure appears. Its not a big deal, I mean, if you're gamin' you're livin'!(tm). I'm not trying to start an edition war or anything. ;) But I think that if you've got a choice in the matter, I think that it'll be more memorable if you make the players earn it. That's just my two cents on the matter, anyway.
But in any case, Game On!
*Both of these riddles are from the excellent "Riddle Rooms #1: Dungeon Dilemmas," by Riddlemaster Games and Cloud Kingdom Games. A highly recommended resource (circa 1993) for your generating great riddle puzzles!