The purpose of the Math Stack Exchange is as a Question and Answer forum for mathematics.
So in regards to riddles, I think most users would agree with the following tests:
- Is the nature of the riddle mathematical? A logic puzzle like knight and knaves is, a clever word play on mathematical terminology ("What's yellow and implies the axiom of choice?") is not.
- Do you already know the answer to the riddle? Math.SE is not a "riddle-me-this" forum, and we discourage the asking of a question (or in this case, a riddle) for the sake of "asking a question". (For example, if your motivation is that "this is a fun riddle and people should see it" or "I want to ask a riddle and judge whether people answered correctly" then perhaps this website is not the right forum.)
- For logic puzzles, have you done your homework? A lot of logic puzzles have long-standing known solutions easily found on the internet. In some cases the solutions have been discussed on this site before.
In your case, a poor way to ask the question would be:
Here's a riddle. What's the solution?
A slightly better way to ask the question would be:
Here's a riddle. Starting from so-and-so we can deduce thus-and-thus that the answer must be such-and-such. But starting from this-and-that we can deduce here-and-there so the answer must be not-such-nor-such. Which is the right answer?
Though most likely in this case it is just that the hypothesis of the riddle contains two contradictory statements.
BTW, since you already asked this question, Dylan's comment is very practical. But in general there is no need to really vet your question before posting it. Just remember that
- A question being closed does not reflect poorly on you as a person; unless of course you refuse to learn from the experience and keep posting similar questions over and over again (which, by the way, would be considered spamming).
- A question being closed does not necessarily reflect that the question, in the abstract, is a bad question: it may just be that the community deems the question unsuitable for this particular forum. (In other words: it's not you, it's us.)
If you can keep that in mind, there's usually no harm to just asking the question on the main site.