People wear a variety of smiles in their lifetimes — some revealing and others concealing.
We smile when we win, and we grin to suppress chagrin. Thus did Herman Melville call the smile “the chosen vehicle for all ambiguities.”
A smile can be knowing, winning or false. It can stretch sly and toothy like a crocodile grin, or appear slight and enigmatic like the smirk on the Mona Lisa. Such a wide variety suggests a hitch in the old saw that it takes more muscles to frown than to smile: Which smile do they mean? A true smile — the kind that involves eye muscles that only 1 percent or so of humans can consciously control — probably takes quite a few more muscles than a frown, while a slight, we-are-not-amused, corners-of-the-mouth upturn takes the tug of only one or two pair.
For the rest of the story: http://www.lifeslittlemysteries.com/3097-facial-muscles-frown-smile.html