SON, TWOS, WENS, and FRY are all homonyms for days of the week, without the "day" part (Sunday, Tuesday, Wednesday, and Friday -- they're also in order). The odd one out is HOURS, which is part of a day.
EXCEED, BOOK, CHOICE, and CODEX are all unchanged if you flip them vertically. The odd one out is ERODE, which has the R.
PLEASURE, DOME, PRONOUNCEMENTS, and RESTRAIN all split exactly in the middle to form two words (plea/sure, do/me, pronoun/cements, rest/rain). The odd one out is STRUMPET, which splits off-center (strum/pet).
PARE, AMPLE, HOPPING, and TIFF all form new words if you add
an S at the beginning (spare, sample, shopping, stiff). The
odd one out is ENCHANT, which forms a new word with P
UNCLEAR, AWRY, NICHES, and POTION all form new words if you swap the first two letters (nuclear, wary, inches, option). The odd one out is WORST, which forms a new word if you swap the last two letters (worts). Yeah, "worts" is a pretty sad excuse for a word. Sorry.
PRECAMBRIAN, CUSTOM, KABOB, and DENMARK all have a male name at the end (Brian, Tom, Bob, Mark). The odd one out is OENOPHILE, which has a male name just before the end (Phil).
POSHER, MILOMETER, BEYS, and MITTEN all form new words if you replace the first letter with K (kosher, kilometer, keys, kitten). The odd one out is ROUGHING, which forms a new word with C (coughing).
YOUNG, USURP, HERITAGE, and HIMALAYAN all start with a pronoun (you, us, her, him). The odd one out is DEFAME, which ends with one (me).
PLY, PECKER, DRIFT, and CHUCK all form words with "wood"
(plywood, woodpecker, driftwood, woodchuck). The odd one
out is STALK, which kind of makes one phonetically
CONDUCT, SUBJECT, PERVERT, and RECORD can all be nouns or verbs, depending on where you put the accent (CONvert, SUBject, PERvert, REcord vs. conVERT, subJECT, perVERT, reCORD). The odd one out is FOCUS, which can also be a noun or a verb, but is always accented the same way (FOcus).
ROOT, CELERITY, CORA, and GARAGE all form new words if you add a B exactly in the middle (robot, celebrity, cobra, garbage). The odd one out is OX, which forms a new word with B at the beginning (box).
CALIBRATE, CHAMELEON, DREARIEST, and PRECANCEROUS all contain astrological signs (Libra, Leo, Aries, Cancer). The odd one out is RESTAURANT, which contains two-thirds of one (Taurus).
PLINTH, SHAFT, CAPITAL, and ECHINUS are all parts of a column. The odd one out is MATRIX, of which a column is a part.
WOLF, REWARD, ERGO, and SNOOPS are all words backwards
(flow, drawer, ogre, spoons). The odd one out is FOOT,
which is almost one (too).
PENDRAGON, MYSELF, ALLEGIANT, and STROLL all end in mythical creatures (dragon, elf, giant, troll). The odd one out is IMPROBABLE, which starts with one (imp).
ALIENATED, CATEGORIZING, REGULATORY, and NATIONALISM are all five-syllable words accented on the first syllable. The odd one out is VITUPERATOR, a five-syllable word accented on the second syllable. (Even if you've never heard the word before, there's no other natural way to pronounce it in English.)
IMPROMPTU, CLASSICS, APPRECIATE, and BENIGN all phonetically end in numbers (two, six, eight, nine). The odd one out is EUPHORIA, which phonetically has a number in the middle (four). The five words are in numerical order.
GERMANY, PLOTS, CHROMOSOME, and CURFEW all end in a quantifier (many, lots, some, few). The odd one out is NEFARIOUS, which almost ends in one (various).
TIBETAN, SYNTHETASE, AUDIOTAPE, and APHID all contain Greek
letters (beta, theta, iota, phi). The odd one out is EQUATION,
which contains one backwards (tau). The five words are in Greek
DEIST, SOBBER, DWINDLE, and CHIEF all form words related to stealing if you change the first letter (heist, robber, swindle, thief). The odd one out is WHENCE, which forms one if you change the first two letters (fence).
HONOR, KNOWN, PSYCHOLOGY, and WRONGED all have silent first letters. The odd one out is SWORDBEARER, which has a silent second letter.
AIRWAY, UNDERPLAY, X-RAY, and ASHTRAY are all words in Pig Latin (wear, plunder, wrecks, trash). The odd one out is ESSAY, which sounds like Pig Latin but isn't.
CHARLATAN, RESERVED, HITCHHIKING, and SUCCUBUS each contain three copies of a single vowel (A, E, I, U). The odd one out is TOPONYM, which has two O's and a Y. The five words are in alphabetical order by vowel.
HOCKNEY, SKIVING, CURDLING, and LUNGE each form the name of a Winter Olympics sport if you delete one letter (hockey, skiing, curling, luge). The odd one out is SCATHING, which almost forms one (skating).
Here they are:
WORST, OENOPHILE, ROUGHING, and DEFAME all have runs of three consecutive letters (rst, nop, ghi, def). The odd one out is STALK, which has a run of two (st).
FOCUS, OX, MATRIX, and FOOT all have well-known irregular plurals (foci, oxen, matrices, feet). The odd one out is RESTAURANT, which only has a regular -s plural (restuarants). Hey, I never said they were all going to be clever.
VITUPERATOR, EUPHORIA, NEFARIOUS, and EQUATION all have one of each vowel (aeiou). The odd one out is IMPROBABLE, which has four out of five.
WHENCE, SWORDBEARER, TOPONYM, and SCATHING all contain animal names (hen, bear, pony, cat). The odd one out is ESSAY, which contains one backwards (ass).
At this point, given five words, surely you know what to do.
Most of the ideas in this puzzle are from the wonderful Unprecedented Discovery, written by Lance Nathan and Francis Heaney for the 2003 MIT Mystery Hunt. And I'm not the only one who thought Unprecedented Discovery was a puzzle concept worth stealing -- Chris Morse and Ann Jones liked it enough to make their own version in 2005 (Odd Man Out, also an excellent puzzle). I took some of their ideas, too.
Thanks also to Jay Lorch, Colin Anderson, David Bushong, and Ann Daniels
for playtesting the puzzle, finding sets that didn't work, and coming
up with ideas to fix them.
Finally, here are two sets that got cut from the final puzzle, but which you might enjoy. (The answers are right below.)
HEXADECIMAL, MONOLINGUAL, TELEVISION, and HOMOSEXUAL are all Greek/Latin hybrids: they each combine a Greek-derived prefix (hexa-, mono-, tele-, homo-) with a Latin-derived stem (decimal, lingual, vision, sexual). The odd one out is RHINOPLASTY, where both parts are Greek. (This is not quite as impossibly difficult as it sounds. "Television" is somewhat well-known for having this property, from Charles Scott's 1936 quote: "The word is half Greek and half Latin. No good will come of this device." "Hexadecimal" might also be familiar, from the widespread story (possibly true) that the word was coined at IBM when boring managers refused to use the logical Latin "sexadecimal". And "monolingual" is ironic.)
MOE, FUTURE, AIR FORCE, and HOLY GHOST all belong to sets of
three (Moe/Larry/Curly, past/present/future, army/navy/air force,
Father/Son/Holy Ghost). The odd one out is SPADES, which
belongs to a set of four (hearts/spades/diamonds/clubs).
--Michael Constant (email@example.com)