This is a behavior called "flehmen". Animals have a secondary sense of smell that causes them to make faces when they use it. The second sense of smell is reserved for those scents that are mating season triggers.
growing up in a medical family, this is how i feel everyday when i listen to NON medical people talk..... "don't go out in the rain you'll catch a cold!" -.- idiot you catch it by crowding inside around already VIRUS infected PEOPLE! stupid.