These words apply to things that are hard to solve, answer or understand. In a limited sense, all are questions, tasks or entertainments that have set answers or solutions and are devised to challenge the wits. A puzzle is a game or contrivance that tests one’s ingenuity or patience. To work out a jigsaw puzzle or a crossword puzzle , one must fit together pieces or words in a certain way to form a whole. A problem is an exercise in learning that tests one’s ability to apply theory, knowledge and technique. To work a problem in mathematics, one must use the given facts to find the missing ones: The problem is to find the average speed of a car on a trip when the distance travelled and travelling time are known. A mystery in this comparison is a story, novel, play or film that arouses one’s curiosity or suspense: a murder mystery . To solve a mystery , one must follow clues and interpret evidence in order to find a plausible explanation for perplexing events. In actuality, a mystery story is both plotted and solved by its author, who must keep the reader guessing until the end.

Enigma , riddle and conundrum all apply to questions or statements designed to perplex. An enigma is a deliberately obscure or ambiguous statement, a dark saying meant to hid as much as it reveals: The Delphic oracle spoke in enigmas . A riddle is a puzzling question stated as a problem to be solved by clever ingenuity. Riddles may be significant, requiring a grasp of metaphor or the ability to comprehend a paradox. The famous riddle of the Sphinx was an enigma that only Oedipus could interpret: "What goes on four legs in the morning, on two at noon, on three at night?" "Man – in infancy he crawls, at his prime he walks, in age he leans on a staff." On the other hand, a riddle may be merely clever, depending on a pun, as: "What is black and white and red (read) all over?" "A newspaper." A conundrum is a riddle that hinges on a pun and involves some fanciful point of likeness or difference between things, as: "What is the difference between a supervisor in a department store and a sailor?" "One oversees sales and the other sails over seas." Conundrum is an adult word for a child’s game – a kind of riddle at best ingenious, at worst inane.

In a broader sense, all these words may be applied to anything that baffles or perplexes. Problem is the most general word. Any person or thing that causes difficulty may be called a problem . A mystery was originally something beyond human understanding: the mystery of creation; religious mysteries . But the word mystery is now freely applied to puzzling things that have not been explained or that are not fully understood.

• Her disappearance has remained a mystery ; Why he went there is a mystery to me.

Riddle is close to mystery in this sense but stresses the idea of eventual solution: the unsolved riddle of the common cold. An enigma is a tantalizing mystery , something darkly veiled or utterly baffling: his biographer passed over his hearty, wholesome public image to probe the enigma of his private life. Applied to human beings, enigma stresses internal contradictions, the presence of opposed traits that make a person hard to understand. Puzzle is close to enigma in this sense but is a less romantic and more pragmatic word, emphasizing the fitting together of pieces to find a solution: Though many tried to fathom him out and to predict what he would do, the leader remained a puzzle . Conundrum , the most specific of these words, is the only one never applied to a person. When used in an extended sense, it means a problem that seems to defy solution but that invites conjecture.

These words mean to confuse or to present difficulty in understanding or solving something. Puzzle may suggest only a mild curiosity and therefore only a mild frustration at the inability to decipher the meaning of something: puzzled by his neigbour’s odd looks. As in this example, the word suggests that enough hints or clues have been picked up to indicate that something is not normal, without permitting one to determine just what is wrong. On the other hand, the word can suggest merely the remarking of a situation which is unusual in some way: puzzled by the friendliness of the strange country’s inhabitants.

Mystify suggests a greater loss of detachment than is true for puzzle and a greater frustration at not being able to get at the meaning of something. By implication, a person must have many more clues to be mystified than he needs to be puzzled; puzzle might also suggest an initial stage of investigation, mystify a later one: first puzzled and then thoroughly mystified as to why she had never mentioned her husband in the hundreds of diary entries she had made.

Where mystify most appropriately suggests astonishment in the face of an unyielding enigma, perplex stresses actual discomfort as the attendant emotion; it may also imply a more personal involvement with its overtones of worry and uncertainty: perplexed by his refusal to tell her where he was going. Baffle , the most intense of these words, combines the sense of astonishment implicit in mystify with the emotional discomfort implicit in perplex . It suggests someone rendered unable to act or venture hypotheses in the face of a strange or inexplicable experience: The odd noises and flashes of light in the empty house completely baffled him.