The ultimate "detection" problem:

There was once a prince, and he wanted a princess, but then she must be a real Princess. He traveled right around the world to find one, but there was always something wrong. There were plenty of princesses, but whether they were real princesses he had great difficulty in discovering; there was always something which was not quite right about them. So at last he had come home again, and he was very sad because he wanted a real princess so badly.