Also, remember when things have been quiet for awhile between rival departments, it's perfectly OK to fill a co-workers purse locker with packing peanuts, knowing she will assume that it was the RIVAL department that had done it.
Then, waunder into the locker room a few minutes AFTER said co-worker opens locker, sympathize, and offer to 'help' clean up the peanuts that have drifted all over the floor - as co-worker is swearing vengeance toward rival dept. - say nothing, cause NOW the games are on again!!