Greenspan, in testimony before a Senate committee on Feb. 16, 2005, observed an anomaly in the economic figures: "For the moment, the broadly unanticipated behavior of world bond markets remains a conundrum."

In the synonymy of perplexity, the choices of nouns were taken over by Winston Churchill, who described Russia as "a riddle wrapped in a mystery inside an enigma." That leaves puzzle, now associated with games, and puzzlement reminds us of the Siamese king in "The King and I." Greenspan could have chosen the participles bewildering, baffling, befuddling, perplexing or even stupefying, but those verb forms functioning as adjectives go too far toward admission of being wholly stumped. The Fed, as an institution, can admit candidly to a degree of uncertainty about the economic future but not adopt an attitude of cluelessness.

Since that first financial use of the unfamiliar word, "the Greenspan conundrum" has become as familiar as his 1996 phrase to describe the frenetic run-up in the stock market as "irrational exuberance." The first step in solving the interest-rate puzzlement is to understand the terminology.

"I must ha' my crotchets! And my conundrums!$(2$) shouted the title character in "Volpone," a famous 1605 play by Ben Jonson, Shakespeare's leading competitor. (That's where crotchety came from, an adjective that descriptive usagists apply to me with increasing frequency and which I take as a compliment.) From the start, conundrum was closely associated with paranomasia. A minister in 1645 was criticized for wordplay in his sermons, "which in Oxford they call conundrums. . .and a pair of dice is made a Paradice." The technique was further derogated in 1704: "Pun and conundrum pass with them for wit." The philosophical anarchist William Godwin, who at first scorned the feminist Mary Wollstonecraft and later married her, wrote in "Caleb Williams," his 1794 novel: "Zounds! Sir, do not think to put any of your conundrums upon me."

At that time of the Enlightenment, the O.E.D. reports, the word gained a sense of "a riddle in the form of a question the answer to which involves a pun or play on words." (That punning riddle, called two centuries ago "a conundrumical question," has a modern version familiar to punning music lovers: "What is the question to which '9W' is the answer?" The answer-question to that conundrum: "Do you spell your name with a V, Herr Wagner?")