As many artists have discovered, the smile is quite difficult to render and can come with an almost unlimited degree of nuance.  Realistic smiles, in particular, which are judged in terms of sincerity and inflection, require an understanding of how to depict tiny differences in detail.

It’s impossible to look at these seaside binoculars and NOT see a face.  Our brains have evolved to detect faces in our environment – vital for survival purposes – even when our rational selves knows that what we are responding to is very unlikely to be alive.  When we are presented with the minimum facial pattern of  two spots above and one spot below, we not only see a face, we assume, as above, that the face is the expression of a sentient being which carries with it a corresponding emotional state; here, a sort of giddy happiness.

The origin of the Hyper-Alert Eye (the stylized character pose where the upper eyelid is continually raised) is ultimately based on the human face. Under certain circumstances when we are both happy and excited (look at the faces of major league athletes who have just made a game-winning hit), the smiling mouth is accompanied by eyes with lots of white above the iris, the result of super-abundant emotional and physical energy.    
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In my book, I classify this expression as the Eager Smile (top, left).  Whatever name you give it, this particular pose has inspired thousands of cartoon versions, where artists have found the smile/wide eye pattern to be highly effective in creating character appeal.  It has been a popular animator’s strategy for decades, with Bugs Bunny (top, center) and Mickey Mouse, the pioneers, and Woody (top, right), a more recent and very successful successor.  ​

I recently read this wall text at the National Gallery in London about two extraordinarily-accomplished 19th c. French painters: "Degas met Ingres in his youth and was told by him to 'draw lines, young man, and still more lines, both from life and from memory.'"

I agree. Drawing in a sketchbook provides an immediate, and satisfying, medium for recording the world, and for experimenting with new pictorial ideas. For me, I like to draw people in railroad stations, cafes and in parks; I like to imagine people I have never met and to animate their faces with lively or deadpan expressions; I also like to use my sketchbook to invent places that have never existed, but which might make good subjects for a more finished drawing or painting.

In this mug shot of a convicted mass murderer, the Hyper-Alert Eye is disturbing and demented-looking; hardly cute and appealing like the matching expression on the face of Tweety, a beloved stylized character.  The essential elements are the same in both faces: hyper-alert eyes (upper lid raised above the iris), slight smile with pursed lips, raised eyebrows and eyes slightly out of alignment (staring off into the distance).  But our emotional response couldn’t be more different.

Photo Personifies GRIEF of Dallas Police Shootings​

Anyone who spends time analyzing faces notices very quickly that we have a double standard when it comes to expression.  When we observe a realistically-depicted human face, we have a fine-grained, rigorous way of determining the mental and emotional state of that face, based on what are often tiny actions in the “hot spots” of the eye, brow, and mouth.  If we conclude, on the other hand, that the depicted face is Sentient, but Not Human (i.e. stylized), we are liable to see almost any configuration of eye, brow and mouth as plausible and readable, as long as there are cues present which indicate an expression (open mouth, raised brows, etc.)