Cat smelling with open mouth?

Cats have sensors that allow them to smell strong odors that we cannot smell, and some of these sensory receptors are actually in the top gum area behind the lip. When a cat opens it's mouth and pulls back it's lip, it is actually forcing or drawing air up into the back of the top gum area. There, the scent is trapped between the gum and the lips, then drawn up into the roof area of the cat's mouth to the vomeronasal organ, a specialized sac membrane with a large amount of blood being supplied to it. After this, the scent is taken from the vomeronasal or 'Jacobson's organ," to the brain, where the brain will interpret the scent molecules, and inform the cat of the origin of the smell.

Most cats use this reaction, commonly known as "Flehming" or the "Flehiming Reaction." It is totally normal, animalistic feline behavior. You will often see this behavior in cats that are smelling urine from other cats, particularly a male cat's urine, which has a very pungent odor. Male cats will often do this to check/test to see whether a female is releasing sex hormones, and therefore in heat (ready to mate with.) This is not only common to domestic felines, but also to their wild counterparts.

Your cat must have smelled the other cats that you were around, and in turn, translated the smells in order to find out more about the cats at your friend's house. A lot can be interpreted from smell: this includes but is not limited to gender, sexual readiness, and age. A lot of cats may become offended after smelling another cat's scent on your skin, and may bite you or hiss. This means that they feel threatened, as you are their owner, and therefore almost seen as property. If a cat rubs it's mouth and body up against you, he or she is essentially trying to "mark" you. We often cannot smell the cat's scent that they leave on us, but other cats can smell your cat's odor on your skin and clothing, and will therefore be able to translate that smell (i.e. "stay away from this girl/boy. He/she is my human, my pride member, HE/SHE IS MINE, ALL MINE! STAY AWAY!" LOL!) This is why most experts recommend washing your hands after handling another person's cat. However it is not only imperative so as not to disturb your cat, but also in the event that your friend's cats could possibly be sick (these germs can be spread to your cat, the same way people can spread colds/viruses to one another.) It is also recommended that you change your clothes or remove your jacket after playing with or being around your friend's cats, so that your cat doesn't perceive the scent of the other cats on your clothing as a potential threat, and seeing you as a "traitor." (As funny as it sounds.)

Lions and big cats even use smells to dictate and mark their territory. This is done by "spraying" (in other words, peeing or urinating,) on trees, grasses, and other foliage. This can go on for miles, and is usually done in four distinct corners of a habitat. Whenever an intruder enters their territory, a pride may move towards the source, hide, then ambush the intruder. Lions can smell the presence of another lion from up to two miles or more away, as well as smell blood from dead prey from this distance (Dead prey typically consists of, but is not limited to, animals in the ungulate family: gazelles, antelope, water bison, etc.) A lion's roar is so loud and distinct, it can be heard from up to 8 km away, and so other felines can hear this call. Most of the time lions roar to give their location to other lions either to draw them in (if they are part of the pride, or are family members,) or in order to warn them that the territory they are inhabiting is theirs and to stay away.

I hope this helps you! :)

Flehmen response in the Sumatran Tiger

The flehmen response (/ˈfleɪmən/; German: [ˈfleːmən]), also called the flehmen position, flehmen reaction, flehming, or flehmening, is a behaviour whereby an animal curls back its upper lips exposing its front teeth, inhales with the nostrils usually closed and then often holds this position for several seconds. It may be performed over a site or substance of particular interest to the animal (e.g. urine or faeces) or may be performed with the neck stretched and the head held high in the air. Flehmen is performed by a wide range of mammals including ungulates and felids. The behaviour facilitates the transfer of pheromones and other scents into the vomeronasal organ located above the roof of the mouth via a duct which exits just behind the front teeth of the anima

This is perfectly normal. Many animals have an organ in the roof of their mouths, the vomeronasal organ. What you're seeing is called the "flehmen response" which allows them to get scents into this organ so they can better smell and evaluate them.

You can find more info here: http://en.wikipedia.org/wiki/Flehmen_response

Cats have sensory glands in their mouths that they use to smell some odors, while it is more common in the wild it is totally normal and you shouldn't be worried at all.

super normal...they do it when they want to smell better.