Part 1 deals with incorporating riddles into an adventure whereby the solution is specific to the adventure. Part 2 is more generic, providing riddles for general use as opposed to a specific use. Part 3 talks about finding new riddles and even provides a way to create your own original riddles.
Parts 1 and 2 provided some riddles for you to use at your whim. However, its likely they won't be enough or appropriate for your own campaign. If that's the case how can you get more riddles to use?
The easiest option is to do an online search for Riddles. It will take some time to sort through riddles that worl and those that do not. Just be careful that the riddles chosen will work in your world.
Q) What goes around the world but stays in a corner?
A) A stamp.
While this is a good riddle, most fantasy worlds don't have a postal system. You need to always remember the common ground you have established with your players. They can't be expected to think of that answer since they know your world does not have a postal system and even if they think of the correct answer they will discard it immediately.
Another easy solution is to head to a bookstore. There are many books with riddles. Again you need to be careful as to your subject matter. While I have never seen a book that is wholely based in a fantasy setting, you would be looking for more general riddles. Sometimes there are subject headings and it would be a good idea to take a quick look at it. If the chapter headings are Riddles about Cars or Riddles about Countries, you should probably move to another book.
There are also riddle books for children and they tend to avoid logic for funny.
Q) What do you call a kitten drinking lemonade?
A) A sourpuss.
It is possible to include such riddles, especially if they are presented by a child NPC, but they are harder to integrate into an adventure. Most riddles are put into place as a test of intelligence or a deterent to those who do not know the answer. Childish riddles are not that serious.
A third option is to create your own riddles from scratch. At first glance this can be intimidating, but we're going to do just that to show you the steps involved and that its not really hard. There are, however, some rules that must be abided by.
1) There can be only 1 solution. If there is more than 1 answer, then it is not a riddle but instead a knowledge question. "What grows in a forest?" is a knowledge question and not a riddle.
2) The answer must be clear. When the answer is revealed there must be a moment of 'Of course that's the answer, I should have seen it.'
We'll now do a step-by-step process of designing an original riddle.
Step 1) Come up with the answer to the riddle. We need to work backwards when designing riddles. This also gives you an opportunity to sculp the answer and riddle to something appropriate to the adventure. In this case the answer will be 'Sand' as a warning to an upcoming trap that causes sand to pour down onto the characters in an enclosed room.
Step 2) Brainstorm all the components (adjectives thereof and places it is used) of the answer you can think of. For 'sand' we have: beach, grains of sand, sandy, small, handful, sand castle, sand in an hourglass.
Step 3) Write up some simple facts based on the components you came up with.
Sand is found on a beach.
Grains of sand make up sand.
Sand is really small.
Its really hard to hold alot of sand in your hand.
Sand castles are made at the beach.
Sand runs down an hourglass to measure time.
Step 4) Look for double meanings. Riddles are often about a solution that has multiple meanings and the trick is to figure out the correct one. Taking one of the sentences we just wrote up and then obscuring it is an easy thing to do. Lets take the line 'grains of sand make up sand'. Grains of sand can lead to grain as used in bread. A line in the riddle may be "It is grain, but can not be eaten". The way it is written gets a listener to think about bread, but then we remove bread as an answer.
An hourglass is always a good choice, "By the hour it runs, but never leaves where it starts". This is a good contradiction which always leads to confusion; confusion in a good way.
Sand castle can give us "You can build a home with it, but can not live in it", another contradiction.
Step 5) Double check the riddle. The line "It is grain, but can not be eaten" is a good line, but some smartguy will think of grain alcohol, which is drunk not eaten. "It is grain, but can not be consumed" removes eating and drinking from the solution.
The hourglass line is tricky but works. "By the hour it runs, but never leaves where it starts". The hour part is a blatant hint but not too blatant. The "never leaves where it starts" is the tricky part. Technically, sand in an hourglass leaves the top part, where it starts, and moves to the bottom part. However, also technically, where it starts can be intepreted as within the hourglass itself. Riddles are all about interpretations so this line will work.
"You can build a home with it, but can not live in it" is a weak line. Bricks are used to build homes and buildings and have a type of sand in them. This pretty much means we can't use buildings or sand castle in the riddle, unless we come up with something really clever but clear. Unfortunately I can't think of such a line. Sometimes its best to drop an idea or line if its proving too difficult to fit in.
And so we end up with:
"It is grain, but can not be consumed.
By the hour it runs, but never leaves where it starts."
This riddle is basically done. I believe there is enough there to allow a player to figure out the answer. The more lines you add the clearer the answer is. If we used just the first line, the answer may be too obscure. There may also be some hidden answer we don't know about. Perhaps there is some type of grain that is not eaten or drunk, maybe because it is poisonous. By adding the second line it reduces the number of possible correct answers. Just be careful about adding too many lines as the answer could become too easy to reach.
Let me know if you try and make your own riddle from scratch. Let us know how the process worked for you.