There also are several handbooks put out by the Mystery Writers of America. The riddle has metamorphed into the mystery. It is a challenge to the reader to solve the problem. Your mystery should have at its heart a paradox that begs a solution. Things are not what they seem on the surface.
Clues lie within the words.
And in the best tradition of the mystery, the answer is in plain view. Pick four characters. Try to make the clues ambigious. Now, decide who is going to try to figure out the mystery. You can have a detective, a school teacher, or whoever.