Season 3: Episode 11 - The Platinum Rule

The Platinum Rule: As Ted prepares for a date with the doctor who's going to remove his tattoo, Barney and the gang try to warn him that he may be breaking a cardinal rule of dating, "The Platinum Rule": don't poop where you eat. Whether it's your apartment building, your workplace or the bar where you hang out, it's never a good idea to get into a relationship with someone you see on a regular basis. Barney uses stories of three such relationships that he, Robin and Marshall and Lily have each experienced to illustrate the nine steps that inevitably follow in any such relationship.