Mary-Kate and Ashley Olsen's smile secret

“Every time they pose and smile, they say the word ‘prune,’” a source close to the girls told OK!.

Another source close to the Olsens says their smiles are keys 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 a full-on pout. It has been the obv(ious) way to tell them apart for years.”

Image WENN