This logic puzzle was created by Raymond Smullyan (author of What Is The Name Of This Book?) and reprinted here: OpenCourseWare on critical thinking, logic, and creativity:
Three gods A , B , and C are called, in some order, True, False, and Random. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random matter. Your task is to determine the identities of A , B , and C by asking three yes-no questions; each question must be put to exactly one god. The gods understand English, but will answer in their own language, in which the words for yes and no are “da” and “ja”, in some order. You do not know which word means which.
While we’re looking for puzzles, here’s one that requires only elementary geometry to solve: World’s Hardest Easy Geometry Problem. Find angle x.
A sliding-tile puzzle. Move the green block to center-bottom. Who knew that the old Wordstar Diamond was still in play?
This graphic puzzle-game involves code-breaking: Notpron, the hardest riddle available on the Internet