This is a popular puzzle in games magazines.  To solve it, you need to 
drop the letters in each column into locations below to form a phrase 
that makes sense.  In this case dropping the letters into place reveals:

A NEUTRON WALKS INTO A BAR
 AND SAYS TO THE BARTENDER
,I WOULD LIKE A BEER. HOW
MUCH IS IT? THE BARTENDER
SAYS, FOR YOU? NO CHARGE.

All you have to do is unscramble the pink squares to read DIRTY RAT.

This was our third version of the puzzle.  Early testers disliked it 
because we gave them a hard version that took 30 minutes to solve, and 
then we overcompensated and ran one that was too easy for the playtest.  
We made this version for the actual game, and it worked just right-- all 
teams finished it in a reasonable amount of time.