We like to think that a smile is always a genuine expression of emotion, but that's really not the case. Discovery News points out that if you want to spot those fake smiles, you don't want to look at the mouth — you want to look at the eyes.
Smiles are easy to fake, and that means we're actually pretty bad at figuring out if someone's happy. Essentially, if you start by looking at someone's smile, you're not always going to get a solid read on the person's emotional state. Instead, Discovery News suggests you start with the eyes, because we're better at reading the emotional state of someone's eyes than we are the rest of the face.
Can You Spot a Fake Smile? [YouTube]