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It's a known fact that the Olsen twins don't eat prunes... or cheese. (Courtesy of shelby temples at Flickr.com)

Stop the presses! The secret to the Olsen twins' smile has finally been revealed! According to msnbc, Mary-Kate and Ashley Olsen mouth the word "prune" whenever they're asked to pose for photogs.

"Every time they pose and smile, they say the word 'prune,'" a source close to the Olsens reveals to OK! magazine.

Another source isn't so clear about what word the Olsen twins use when taking photographs. However, the source does confirm that their smiles are key to their identity. "It could be true (that they say prune) but unfortunately, I can't say – they like to keep their personal lives personal," said the source. "They do care about their smiles though. Mary-Kate has slightly thinner lips and more of a playful grin, while Ashley tends to go for the full-on pout. It has been the obvious way to tell them apart for years."

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