In a good mystery, it's hard to tell which road the author is leading you down. You think you've figured it out, but then the plot suddenly curves in a different direction. Sometimes you don't know where you are at all.
It's not easy for an author to do this. It involves not jumping at the first solution that comes to mind, but instead delving deeper, going for the fifth or sixth. Just make sure whichever one you choose makes sense or the reader will feel cheated. You may need to go back and plant clues to back up your choice. It's not enough to make sense, you also need to make the solution not too obvious or it won't really be a mystery. You've got to keep them guessing.
Can you tell where this photo is from? Let me know your guess. You can see if you're right tomorrow when I post the answer on my daily blog, http://morganmandel.blogspot.com/ .
Or, if you don't have a clue, instead maybe you could share the name of a mystery where you were surprised by the author. Or, maybe you could share ways you've surprised readers yourself when writing your books. Please share.