Have you ever noticed this weird cat behavior? Your cat greets you from returning home, sniffs you, and suddenly freezes with a strange look on their face. Mouth open, breathing in with their teeth bared or perhaps it looks almost as if they are panting and grimacing. Don't be scared, your cat is fine and I promise they're not angry at you. What you are seeing is called the flehmen response.
The Jacobson's Organ
Your cat doesn't just have incredibly heightened senses which allow them to see and hear and smell better than us. In fact, your cat actually has a whole other sense. Not a sixth sense, exactly, more of an organ that allows cats to sense information that is a cross between taste and smell. Your cat uses nasopalatine canals, which are two small ducts that are located in the roof of the mouth of your cat. The nasopalatine canals then carry molecules to the vomeronasal organ. The vomeronasal organ (also known as the Jacobson's Organ) is a sensory organ located in the nasal septum.
The reason your cat is making that strange face is that during the flehmen response, your cat is using their tongue to flick the scent and molecules towards the ducts. They open their mouth (baring their teeth) for maximum exposure so they can access the most scent possible. The ducts are full of saliva, so the nasopalatine canals act as a pump to the vomeronasal organ using special muscles. It is an involuntary act for your cat.
The Flehmen response
This organ is actually common in several mammals: zebras, elk, giraffes, llamas, rhinos and even dogs all have the flehmen response. However, cats have some of the most powerful vomeronasal organs. While a hound dog has 9 receptors in their vomeronasal organ, a cat has 30 different types of receptors. So when they have the flehmen response, it's extreme sniffing going on!
Most of the time cats are using their flehmen response to investigate smells from the same species. So if you have another cat in the house, there is a good chance your cat is doing it more. The vomeronasal organ is especially important to your cat for marking territory, mating and intraspecific communication (communication between the same species).
If you don't have another animal in your house, this can still happen if you carry the scent of another animal on you. So your cat is trying to figure out who else might have rubbed their tail on you during the day. Your cat can also have the flehmen response to your scent when it is more intense. For example the dirty laundry you have left around the house might be of interest to your cat especially because those items are particularly heavy with your scent.
The wild thing is early humans used to use a flehmen response too! In utero humans still start developing a vomeronasal organ but it fades away during the growth process and is now a vestigial organ. You can still feel a couple of pits under the nostril where the ducts used to connect to the vomeronasal organ.
So if you're worried about your cat's weird, faraway-looking grimace, don't be. your cat is just fine. If you are concerned about your cat's sniffing behavior, you can look for some telltale signs that your cat might need a trip to the vet. But having the flehmen response is no need for concern. They're just trying to get all the information they can.