If the puzzle could have multiple answers or the puzzle is subjective or opinion-based, do not post it here. This includes riddles, etc. The user shouldn't have to come back and say: "Hmm, that works, but it's not the right answer. That other one is right!"
If the puzzle is generic or can be trivially modified, do not post it. For example, "Solve this hangman puzzle: P _ Z Z _ E" is bad because it's unlikely to help future visitors and it's way too narrow, but "What's a good strategy for guessing vowels in specific words in hangman?" (for example) is good.
If the puzzle is reasonably unique and could help others in the future, post it here! For example, "How could I move three coins in this triangle to make it point the other direction?" (I just picked an example of a fairly well-known puzzle for this, but there are probably much better examples out there)