Of all the arcane corners of academia—comparative puppetry, the history of flags (officially called “vexillology”)—perhaps my favorite is riddle studies. And of all the corners of riddle studies, perhaps the liveliest is the study of medieval riddles, specifically 95 riddles found in The Exeter Book, a 10th-century volume compiled by British monks.

To research my new book The Puzzler, I called one of the stars of medieval-riddle studies, Megan Cavell, an Old English lecturer at the University of Birmingham who runs a Web site called the Riddle Ages.

Cavell says she discovered medieval riddles partly through her love of Old English, the language in which they are written. “It’s just the most beautiful, guttural language,” she tells me. “Sometimes I’ll shout bits of Old English at my partner. If I want a glass of water, I might say that in Old English.” (It’s “Gief me wæter,” in case you were wondering.)

Cavell has spent much of her time studying The Exeter Book, which is well known for two reasons. The first is that a handful of the riddles are really, really naughty. Consider the infamous Riddle No. 25 (part of which is translated here into modern English):

My stem is erect, I stand up in bed,
hairy somewhere down below. A very comely
peasant’s daughter, dares sometimes,
proud maiden, that she grips at me,
attacks me in my redness, plunders my head,
confines me in a stronghold, feels my
encounter directly,
woman with braided hair. Wet be that eye.

The answer isn’t what you’re thinking. It’s an onion! The eye is wet from crying, not from anything X-rated. A classic misdirect.

The riddles raise the question: How did celibate monks get away with sexual references?

Cavell says that riddles were a “safe space where you could explore taboo topics.” The format gave monks plausible deniability. “If you solve it wrong, if you solve it sexy, then bad on you,” she says.

How did celibate monks get away with sexual references?

Many of the riddles in The Exeter Book are sexual, such as one that hints at a leather dildo but is allegedly about a horsewhip. A few prudish scholars in the Victorian era were so horrified by those types of riddles that they refused to translate them, condemning them as “smut and horse-laughter.”

Cavell and her colleagues have no such qualms. Along with some other academics, she is looking at them from a feminist perspective. Cavell has even researched the possibility that these words were secretly written by women—or, at the very least, were influenced by women.

The second reason that the Exeter riddles are well known is that we can never be sure of the true solutions. There’s no answer key. Some answers are obvious (such as the onion), but others are a matter of debate. A lot of debate. As in thousands of hours of debate by dozens of scholars hashing it out in journals and conferences.

Consider Riddle No. 4. Based on the amount of bickering it has produced, it might qualify as the hardest riddle in history. Academics have proposed at least 13 solutions, ranging from a bucket of water to the devil to a phallus. (Yes, again with the smut and horse-laughter.) Here it is, in case you want to try your hand:

At times busy, bound by rings,
I must eagerly obey my thane,
break my bed, proclaim with a cry
that my lord gave me a neck-torque.
Often a man or woman came to greet me,
sleep-weary; I answer them, winter-cold,
the hostile-hearted ones. A warm limb
sometimes bursts the bound ring;
however, that is agreeable to my thane,
the half-witted man, and to myself,
if I could know anything, and tell my story
successfully with words.

Most people in the puzzle community want definitive answers and closure, not ambiguity. Cavell, meanwhile, says she doesn’t mind the fact that she’ll never know the real answer. On the contrary, she says, “I actually prefer the riddles that have five solutions that all work just as well as each other.… I love the multiple truths.”

A. J. Jacobs is a journalist and the author of several books. His latest, The Puzzler: One Man’s Quest to Solve the Most Baffling Puzzles Ever, from Crosswords to Jigsaws to the Meaning of Life, is out now from Crown