Roxanne Chalifoux (aka Villanova Piccolo Girl) drew on our national empathy when, despite her Basketball team's March Madness loss - she played her piccolo through her tears. No fear .... Jimmy Fallon and The Roots to the rescue!
A little-discussed component of a sincere smile which is extremely subtle but also present here - is a dynamic lowering of the entire width of the eyebrows.
The mouth is not required to be opened in order for the smile to be a sincere one - yet a fully expressed sincere smile does indeed display the teeth - however only the upper ones (exceptions here are when there is a camera angle is looking downwards [or if the observer is relatively taller to afford a similar angle], if the smile was sincere - but through fatigue is losing its vigor, or if the smile is transitioning from or to sincere laughter [sincere laughter may display both upper and lower teeth]).
An interesting additional corollary is noted here with respect to Ms. Chalifoux's blushing. There often tends to be a persistence of blushing - or often an in increase - during moments of suppression of mouth smiling. This is a form of what in body language parlance is termed "leakage" - when attempting to suppress an emotion by not displaying one nonverbal component (e.g., here the full mouth element of a smile) - another nonverbal component (e.g. in this example blushing) persists or grows stronger.
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