According to Dacher Keltner, a researcher at University of California, Berkeley, our facial muscles have the ability to make two kinds of smiles. One he terms the "Pan American" smile in which only the muscles at the corners of the mouth go up. The other, called a Duchenne smile, is when the muscles around the eyes are involved, the kind that make "crow's-feet." These muscles are mostly involuntary and in addition to crow's-feet, a Duchenne smile gives you a little gleam in your eye, raises your cheek up and impouches the lower eyelid. Also according to Keltner's research, when people see a picture of a Duchenne smiler, even when it's presented subliminally, it makes you smile in return, and feel calmer, more relaxed. The Duchenne smile is considered the most heartfelt smile, because it is linked to feelings of happiness and activation of the left hemisphere of the brain, which is associated with positive emotions. "Young infants show it when their mom approaches," notes Keltner.

This is the kind of smile that Clay gives us most of the time. Sometimes it's accompanied by giggles. Sure, once in a while he'll do one of those Pan Am smiles just when he's being polite, but most of the time it's those full on Duchenne smiles.

And we can't help but smile in return!