We know why politicians lie when they get in trouble: they think the consequences of telling the truth are too severe to bear. That's why Richard Nixon lied about Watergate, and Bill Clinton about Monica Lewinsky. The more complicated question is why they fib—why politicians insist on stretching unimportant stories in ways that are easy to check and refute. Hillary Clinton's oft-told yarn about ducking sniper fire on the tarmac in Tuzla, Bosnia, in 1996 has gotten a lot of publicity, maybe too much. Her misrepresentation of her role in the Northern Ireland peace talks was more serious but less visual on YouTube. Even so, the Tuzla Tale tells us something about her insecurities and frustrations, which in turn helps explain why she's losing.

Everyone tells fish stories once in a while, exaggerating a tale for dramatic effect, and if they claim they never have, well, that's a fish story in itself. But there's a difference between telling your Aunt Mitzi that you caught a fish that was two feet long when it was actually 15 inches, and falsely claiming you wrote the Haitian Constitution (vice presidential candidate Franklin D. Roosevelt in 1920), or had helped liberate Nazi death camps (President Ronald Reagan to Israeli Prime Minister Yitzhak Shamir and separately to Nazi hunter Simon Wiesenthal in 1983).

Politicians by nature construct narratives about themselves, polished personal stories that make them shinier, if not larger than life. And the good ones start the self-mythologizing early. Barack Obama's memoir, "Dreams From My Father," written before he entered politics, includes a few composite characters and disputed details (e.g., co-workers say he didn't have his own secretary when he worked at a company in New York). By some accounts he may have even exaggerated the extent of his youthful drug use to make himself seem more troubled than he really was.

Over time, the movies that politicians create in their heads become real to them. In that sense they lie, as was said of Henry Kissinger, not because it's in their interest but because it's in their nature. When certain buttons get pushed, the projector turns on and the fantasy begins.

Hillary's Bosnia whopper traces back to 1992, when she angrily told reporters who questioned her role in her husband's career that she hadn't been staying home "baking cookies and having teas" but was instead out working. This was in apparent reference to her 15 years as a corporate lawyer for the Rose Law Firm in Little Rock. On the trail she now emphasizes her "35 years of public service," which, because it presumably doesn't include corporate legal work, is a reference to her extensive nonprofit activity and to her time as First Lady of Arkansas and of the United States. While Hillary is not generally an insecure person, she is highly defensive about this record. Without it, she would have only seven years (her Senate career) of public service to cite. Hillary's "movie" of her own life, and of her presidential campaign, is dependent on those years seeming as meaningful as possible. An assault on their importance cuts deep.

Late last year, Obama began to push that button by belittling her travel to 80 countries as First Lady. He said that "having tea" with foreign leaders wasn't the big-time foreign-policy experience she claimed as a major reason to elect her. So Hillary began to tell the Tuzla story, which had first appeared in her memoirs, in a more dramatic fashion. "I don't remember anyone offering me tea on the tarmac there," she said, firing back at Obama. Michael Dobbs of The Washington Post fact-checked the tale in March and gave it "Four Pinocchios." It turned out to be untrue in almost every detail: no "corkscrew" landing, no sniper fire, no canceled airport reception. (She and Chelsea were greeted by an 8-year-old Bosnian girl, among others.) Sinbad and Sheryl Crow had come along to entertain the troops and saw nothing scary. Even the idea that she was the first wife of a president to go into a war zone was wrong. Both Eleanor Roosevelt and Pat Nixon had ventured closer to danger.