Yep, it's another one of those posts designed to infuriate your players. This time around, I'll make a few observations concerning riddles (obviously); namely, how to use them, when to do so, and how they can be solved.
Riddles, like mazes, can pose a great mental challenge to your players, and have a knack for eating up time. Though they're somewhat easier to circumvent/avoid than mazes (depending on the scenario, of course), this also makes them far easier to place and run than a standard labyrinth. Many of the rules for utilizing mazes are compatible with these puzzles, so I'm not going to linger on this topic.
Much like 'How', the 'When' of riddles is similar to mazes, save for certain considerations that should be taken. Riddles, obviously lacking the mechanical depth of mazes, are difficult to center an entire session or campaign around. Having a singular, inherently non-threatening challenge can be difficult in general to work with, which is what makes the context of this challenge so important. To combat this factor of passivity, the knee-jerk reaction of many Hosts is the placement of a severe punishment for those who fail to complete this challenge (namely death or dismemberment). I, however, feel this to be rather draconian, and a good way to frustrate an uninspired group with endless hours of desperate pondering. Personally, I prefer to have the riddle deny access to an area entirely voluntary and inconsequential to the plot. This optional area can be as rewarding as a shiny, new weapon, or an alternative, safer route to a dangerous objective.
Most people (that I'm familiar with, in any case), tend to strictly accept a valid answer for their riddle of choice, and will yield to no other solution. This method, unfortunately, has the drawback of immediately ceasing game play, especially if the group one is working with are particularly unimaginative. To remedy this issue, it's becoming increasingly common practice to allow a roll or check to "find" the solution, allowing the fictional character to solve it in-game without player intervention. The drawback of this method is its simplicity, which allows even the most complex of questions to be answered with a single lucky roll (in the case of the unimaginative group, wouldn't be a drawback at all, if it preserved the session's momentum). With this issue in mind, the principle of keeping strictly optional areas guarded via riddle is further validated, as failure to access said zone shouldn't have a relevant effect on the overall plot/session structure.
With the discussion out of the way and without further ado, I present a handful of sample riddles to get you started (as always, feel free to modify them any way you see fit):
1. Two doors before you, each guarded by a spirit
One always tells the truth, the other always tells a lie
There's no way to discern between the two, or to tell which one will lie
You also know that one door leads to certain death; the other to your goal
They permit you to ask a single question, directed at one spirit, before you choose a door
What do you ask?
2. A murder was committed and 5 suspects are being questioned
Suspect #1 says Suspect #2 did it
Suspect #2 says Suspect #1 did it
Suspect #3 proclaims their innocence
Suspect #4 says Suspect #5 did it
Suspect #5 says Suspect #4 is lying
If only one of these men told the truth, who spoke truthfully and who is the killer?
3. Two bodies have I, though both joined as one
The more I stand still, I quicker I run
What am I? (My personal favorite)
4. Alfred is the father of 5 children
Half of his children are daughters
How is this possible?