WESun Puzzle Challenge given November 22, 2003:

When lower case letters are squeezed together, the consecutive letters RN (rn) might look like M (m) RM (rm)[sic]. For example, in a quick reading, churn (CHURN) might look like chum (CHUM). What common word ending in RN becomes its own opposite when you change the rn (RN) to m (M)?
-- WESun Puzzle, 2003-11-22

rn m

The online website that I quoted had a typo, they mean "rn" looks like "m".
Boston.pm caught it though!

The following brute-force approach works for the small dictionary.

$ perl -nwle '$w{$_}++; 
  END{while (($k,$v)=each %w) {
	  my $wm=$k; 
	  next unless $wm=~s/rn$/m/ && exists $w{$wm};
	  print $k," ",$wm} }'		 /usr/dict/enable1.txt

Obviously, could reduce space by only memorizing /m$/ and /rn$/ ; and could use prior trick of searching for conjugate during main pass:

> perl -nwle "next unless /(?:rn|m)$/; $w{$_}++ ; my $m=$_; 
	next unless $m=~ s/(rn|m)$/length($1)==1 ? 'rn' : 'm'/e; 
	next unless exists $w{$m}; 
	print $_,' ',$m;" 		/usr/dict/words

burn bum
churn chum
darn dam
stern stem

The amusing thing is that some people will try to say "don't give a darn" to avoid giving offence, while, when if spelt properly with the original reference, "wouldn't give a tinker's dam" should not be offensive. But that's not the one they wanted.

enable1 has some more words, most only useful in Scrabble^®:

airn aim
barn bam
burn bum
carn cam
churn chum
curn cum
darn dam
ern em
fern fem
hern hem
modern modem <-- synonyms once, antonyms today?
morn mom <-- Mornings belong to mothers?
porn pom
sharn sham
stern stem
tarn tam
torn tom
urn um
yarn yam

Webster's 2nd word-list (web2) has even more Scrabble words.

alburn album
arn am
barn bam
born bom
burn bum
churn chum
curn cum
darn dam
dorn dom
durn dum
garn gam
girn gim
harn ham
hern hem
kirn kim
learn leam
scarn scam
sharn sham
starn stam
stern stem
sworn swom
tarn tam
turn tum
urn um
yarn yam

Which one is it?

And what has it to do with Uri?

~~~~~_d( stern stem /^/~~~~~