Four friends, Edwin, Fred, Gary and Howard, were playing together when one of them broke a vase. The teacher asked,~"Who is the culprit?"  \textbf{Both Edwin and Howard said, "Not me."}  \textbf{Fred said, "Howard broke the vase."}  \textbf{Gary said, "Fred is the culprit."}  If only one of four boys was lying, who broke the vase? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
Either Fred or Howard must be lying since what they said is conflicting. Since only one person was lying, Gary was telling the truth, which means Fred broke the vase. 