A smile, one of the most basic elements of human interaction, says so much about how we feel inside. Wikipedia notes, "A smile is a facial expression formed by flexing the muscles near both ends of the mouth. The smile can also be found around the eyes. Among humans, it is customarily an expression denoting pleasure, happiness, or amusement...Cross-cultural studies have shown that smiling is a means of communicating emotions throughout the world. But there are large differences between different cultures. A smile can also be spontaneous or artificial (when people feel obliged to smile). Happiness is most often the motivating cause of a smile." (http://en.wikipedia.org/wiki/Smile)
But can you spot the difference between a real smile and a fake one. Try this psychology test from the BBC website: http://www.bbc.co.uk/science/humanbody/mind/surveys/smiles/index.shtml.
I'm happy to say I got 18 out of 20. Let's see if anyone can beat me.