Office pranks have a long and distinguished history. You've probably seen this one before, but in case you haven't, this video will show you how to do the upside-down coffee cup prank. Simply take a full cup of coffee, place it on the desk of your victim, place a piece of paper flat on top, then carefully flip the cup onto the desk and remove the paper. The cup will be upside-down, full, on the desk. The only way to remove it is to soak oneself with coffee, as your victim will most likely do.
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