Between the 2 smiles

There are two types of smile

1. when human in situation where they think I wamt to smile, the brain frontal cortex which deal with emotion takes delicate control over 50 or more of facial mussles to create a natural smile
2. If you force yourself to smile, the locomotor center send the order "SMILE" to control the muscle. and since forcing action is a base action, the smile always end up loking awkward.
   

that is the only two major types I found in Mr Brain (EP7). It is true though, even you could feel how awkward yoour smile is and people could detect it from your eye. Hence the most beautiful smile is a smile that coming sincerely from your heart and it is reflected trough your whole face. Quite hard is it? But be Happy and it will automatically appear in your eyes.
Ogh and one more thing, when our ancestor, primates, felt something is approaching them in the jungle, they would always assume that it was an enemy and act defensively. Then in the moment they met, they would bare their fangs and punce. If they realized it wasnt an enemy, they would halt their pounce but they wouldnt have time to cover their fangs. Since leaving that would be bad, in order to show they werent hostile, they would smile. And this is the original form of smile.