Apparently that puckered smile that makes the Olsen twins’ seem engaged ("We’re happy to be here!") yet reserved ("Teeth are so crass!") has a name. It’s called "the Prune."
According to MNSBC, a source told OK! that "Every time they pose and smile, they say the word ‘prune.’"
Another unnamed source refused to confirm, but said, "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 obvious way to tell them apart for years."
That explains it! Something tells us that if we ever actually tried Tyra Banks’ "smile with the eyes" and "the prune" at the same time, our facial muscles would simply give out.