Cats will open their mouths after smelling smelly feet. This is an instinctive reaction called the cleft lip sniffing reaction. Most cats will have this behavior after smelling the smell. Cleft sniffing itself is a reaction to identify the hormones contained in the urine of the opposite sex, but when smelling a pungent odor, cats will also have a cleft sniffing reaction, so after smelling smelly feet Will open his mouth.