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. The easiest way to start is to figure out things you know must be true. In the first column, the first line has a one letter word. The only letter in the first column that can be a one letter word is "A". You can continue to do this and eliminate letters that don't make sense, and eventually you can drop all the letters into place and reveal:

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.