Have you ever seen
A sincere smile?
Yup. How on earth can we know the smile is genuine or just a plastic glued on the face?
Well, I don't have the concrete answer for sure. However, we can 'see' the sincerity emitted by those smiles (note: not referring to teeth).

I'm not sure if you've ever notice, but sometimes we can know the caller's expression just through the caller's tone. Yup. The normally spoken "i am fine",can be threaded with more than just one possible 'real emotion'. And, just who would know the real sound of their emotion?
Yup!
Who would?
You can be smiling up to ears, but with rains flooding around your day. And yet, you could be so emotionless even if you're actually is jumping over the moon!
Then, just who would know???

I love to quote Luqman's will upon his son. How he taught his son of The All Aware one by giving such an analogy. Yet, the analogy suits a lot for us to grab the big picture of "the one that knows the most, of everything, all the time"

Seriously, who would bother taking care of a mustard seed grain?

Allah is.

He cares even if the feeling that you fold inside yourself is not known by others. Then, no matter how hard you try to hide the rains, He knows your heart is wet. No matter how emotionless you are in pouring your emotions, He sees you're jumping out of excitement. He just knows.

Well, smile has thousands faces. If you just happen to know your friend's smile isn't genuine, then help him/ her to smile sincerely. ( note: tickle him/her)

Let's appreciate the genuine smile!
Life's tooo short for faking a worthful smile :)
Sense it!