09/29/2011 08:04 am ET | Updated Nov 29, 2011

Anna Faris: Why I Stayed A Virgin For So Long, Talks 'What's Your Number' (VIDEO)

It's hard to tell what was funnier last night: Anna Faris' retelling of her tortured years, or David Letterman's complete lack of preparedness.

It is hard, of course, to host a nightly show, with two new guests per evening; there's always new names, careers and pre-planned conversation to remember. Still, he was pretty brazenly unaware of even one fact about Faris, one of the top female comedy actresses and star of this weekend's romcom, "What's Your Number."

Upon her sitting down next to his desk Tuesday night, Letterman immediately asked her life story. May have been a planned bit, true, but it's likely that he wouldn't have been reading directly from his notes -- on camera -- to get to the payoff anecdote to which that question led. Had he done some pre-reading, he also would probably have known that her husband, Chris Pratt, is not her love interest in the new film; instead, he'd have known that that role went to Chris Evans. And then, he probably would have been aware that Evans played Captain America this past summer, not Wonder Man.

But because he's David Letterman, the late night host of choice, Faris had to play along. Luckily, she was game, laughing at the mistakes and telling her stories in the normal woe-is-me, I'm-actually-a-dork leading lady fashion. To be fair, there was something more authentic seeming about her story of frozen yogurt woe.

Faris told Letterman that, as a chubby teenager who looked like she was eight years old, her parents had her cast in a frozen yogurt commercial, pitching a fat kids alternative to ice cream. She never heard the end of it, she said, and her braces and other dental contraptions made her a class A dork. It was a perfect strategy, Faris revealed, to keep her and her sister virgins for a very long time.

Of course, in her new film, Faris plays a woman who has slept with twenty men and is afraid of adding another to her number. So maybe she should have kept the braces for the flick, too?

She then told Jimmy Fallon another story of young ineptitude on Wednesday, talking about a Pink Floyd song and a sad breakup.