I really love riddles. I think riddles are an excellent way to keep your mind sharp and can provide entertainment for hours when you're trying to solve them in a group. A riddle is a question intentionally phrased to require ingenuity and thought to ascertain its answer. It usually has a logical conclusion but it is usually not obvious at first. For example, there's this one, that simply asks how many oranges you have when you take away two. The immediate response by most is that you have one left - but this is wrong. Once you hear the answer it's immediately obvious that it's correct, even though it may feel a little bit like a 'trick'. Because the answer isn't obvious at first, that is what makes riddles fun.
Solving a riddle without help can give you a great sense of accomplishment and can make you feel like you have a slightly higher level of mental prowess for having done so. This is why at RiddlesAndPuzzles.com, each new riddle is presented first without an answer for a period of time before the answer is finally revealed.
Another meaning for 'riddle' is to 'make many holes in someone or something.' You'll often hear it this way in TV shows or movies as something like "it was riddled with bullets". I believe this meaning of the word comes from the fact that a 'riddle' is also a large, coarse sieve used to separate sand from gravel. Since it would necessarily have a lot of holes in it, it lends itself to the verb form of making holes in something.
These are a few of my favorite riddles, in no particular order: