Do you have a male cat at home? If you do, you have probably seen him lift his head, curl his upper lip back and wrinkle his nose similar to the kitty in this picture. I have certainly seen it numerous times, not only in my own cats, but in cats I have cared for over the years.
This grimace actually has a name. It is called the Flehmen Response or the Flehmen Reaction named after the German verb " to curl the lip". This grin of sorts, is a reaction to a scent the cat has encountered, usually the urine of another cat, but it can also be triggered by other scents such as catnip. The cat is actually smelling pheromones given off by another cat to determine it's gender, it's health and its reproductive status. ( ie. female cat in estrus). By opening his mouth and tilting his head back, the cat is transferring the scent to the Jacobson's organ which is located on the roof of his mouth. That is where the information is analyzed telling the cat the information he needs to know about the origin of the smell.
It is quite an amusing face to see on a cat. Although it can sometime appear aggressive, it is perfectly natural and peaceful. Some other species of animals that display this response, are horses, zebras, tigers, lions and a few others.