What word can you make out of the letters iooulnst - it can only be one word?
The solution to this riddle is a synonym for answer. Or, you could say that the letters iooulnst can form a word which is not "answer," but actually means "answer." The solution to this riddle is also an antonym of "question."
The solution to this riddle has a second definition. It can also be defined as a homogenous, molecular mixture of two or more substances. Examples would be sugar dissolved in water.
If you are still having trouble solving this problem, you could look up synonyms for answer. Riddles often have metaphoric or allegorical answers. Riddles also can put the answer in the question by way of a double-meaning or a pun. Lewis Carroll was famous for this and wordplay has actually been utilized to tackle philosophical quandaries such as Russell's paradox.
Here is another example where the answer is actually in the question: Using the letters in "new door," form one word. There is only one possible answer.
The answer is: "one word."
The answer to your riddle is: "solution."