This is a normal reaction. Because cats have Jacobson organs. Cats have a feeling that humans don't have, that is, a feeling between taste and smell, and a unique sensory organ, that is, Jacobson organ. The organ is connected to the top of the mouth (maxilla) through a pipe and transmits sensory information to the sexual center of the brain. When a cat sniffs a smell with Jacobson's organs, it opens its mouth slightly.