While at a party that is serving a large amount of alcoholic beverages such as beer, one or more individuals urinates in an empty, or slightly filled beer bottle without anyone looking. The bottle full of urine is then placed in the back of the refridgerator to ensure that by the time someone finds it, they will be fairly drunk and wont immediately notice that it is full of piss and take a sip of it. This can be done with various other beverages but beer seems to be the best to use to pull this off because of the large numer of beers at parties and many bottles are colored and thus, makes the color of the urine less noticeable.
We pulled a beer swap at the party last night. The guy who got it was so drunk that he didn't even know it was piss until he took 2 sips. After he found out though he puked on some chick's face.