Many lolcats and other memes use images of animals displaying the Flehmen response. Because we tend to anthropomorphize animals, we associate this response with similar hilarious human facial expressions. But what is it?
The sense of taste and smell are closely linked, and the Flehmen response is effectively smelling through the mouth. The response closes the nostrils, allowing pheromones to be drawn in through a duct behind the upper front teeth of animals, into the vomeronasal organ.
Here's a cat displaying it while investigating pheromones emitted by another animal:
And here's a large cat doing the same:
White Tiger by Flickr user Tambako