From the Cambridge dictionary "smile" is a happy or friendly expression on the face in which the ends of the mouth curve up slightly...But are we sure there is nothing more then that? By my traveller experience I might say that I have met and talked with so many people that count and remember is impossible but even that, they all had something in common, the Smile. I consider it part of an universal language, something extraordinarily powerful able to change (in better) the day of a person. It can gives you positive vibe and see things in a different way, it can be used as a destrees, it can be use to make you happy, laugh, carry on and I believe many other purpose plus: "It's free". And you: "Do you smile?"
Ops I forgot to tell you that smile helps to smile!