"I cannot tell a lie" is a lie we cannot tell...

From the website, The Phrase Finder:

Q: Did George Washington chop down a cherry tree?
: A: Probably not. The story was likely invented by a man named Mason Weems shortly after Washington's death. Ironically, the story was intended to show how honest Washington was: George confesses to his father saying, "I cannot tell a lie."

: From http://www.virginia.edu/gwpapers/faq/index.html More about the fable is at http://www.virginia.edu/gwpapers/documents/weems/index.html

Parson Weems was a man bent on the Moral Uplift of Children, so he wrote a fictionalized biography of America's first president, including a number of fanciful stories intended to polish George's reputation. He succeeded so well that the book was a staple of American education for much of the 19th century, and the legends took root. Today, in a more skeptical age, we tend to dismiss all legends and reduce all historical figures to their all-too-human ordinariness. The story is dying out, in other words. I think it's only older Americans who recall the "I cannot tell a lie" story. The ironic thing is, George doesn't need the help. Although some historians would disagree, he's a pretty admirable character in many ways. For example, in how many revolutions, before or since, has a leader won two elections, then at the peak of his popularity, refuse to run for a third term, voluntarily stepping aside?

Well, if Washington didn't tell a lie, then he was the only president who didn't. Presidents usually keep big secrets, and aren't above telling whoppers when the facts would compromise national security, cost them an election or even personal embarrassment. "I did not have sex with that woman, Miss Lewinsky."

The truth is we lie all the time. "How you doing?" "Great!" Actually, you may be not doing well at all, but this lie is so much a part of us we don't think of it as lying. We don't like to be lied to, but we don't think twice about lying our way out of a situation. "No, honey, that skirt does not make you look fat." "I only had one beer, officer."

So if George Washington, even in a fable like the cherry tree story, was that morally upright that he could not tell a lie, then he wasn't human. Show me someone who doesn't lie and I'll show you someone who can't be trusted!