Again, the riddle:  

One of Alan, Bill, Clyde, and Dan ate all the ice cream. If only one of the following statements is true, who is the culprit?

Alan: "Bill ate all the ice cream"
Bill: "Dan did it"
Clyde: "It wasn't me."
Dan: "Bill lied."

Answer:  Clyde ate all the ice cream, and only Dan told the truth.