Heh heh, sorry. That's always my delighted reaction when I see any cat make this face. I've amused countless friends and family members over the years by shouting out "UGLY FACE!" anytime I see this peculiar and hilarious feline response to an unrecognized smell.
Apparently it's a thing called the Flehmen response, a.k.a. "flehming" (I prefer it as a verb) and there's an actual scientific explanation for it. The origin of the word is the German flehmen -- "to bare the upper teeth." The purpose of this action is to expose the vomeronasal organ, located on the roof of the animal's mouth. Curling the lips in this fashion helps draw scent molecules into the organ, helping the animal to identify the smell.
|Cool science-y diagram|
Apparently cats aren't the only ones that do this. An image search for "Flehmen response" turned up these (awesome) photos:
Whether or not the Flehmen response is a real thing in humans is up for debate. However, I did find some photographic evidence of a certain homo sapien clearly exhibiting some flehming-like behavior...
|I don't want to know where that hand has been.|
(Just kidding, Billy)
Also, I happened to find this very funny post by another flehming-obsessed blogger who conducted his own flehming experiment in the workplace.
So now you know the fascinating explanation for The Ugly Face. And knowledge = power.