This is Emma:
Despite being less cute than any dog, she is okay for a roommate's pet. Emma is a fan of her treats and catnip, both which come in a crinkly, resealable container. Whenever she hears this crinkling, she is near you within seconds. Emma is a conditioned cat.
She has heard the crinkling of the bag so many times, a crinkling with normally follows a treat of some sort, that she is conditioned to react to it. The CS (conditioned stimulus) in this case is the crinkling noise of the treat and catnip bags. Emma's CR (conditioned response) to this stimulus is salivation, similar to Pavlov and his dogs. The UCS (unconditioned stimulus) is her treats or catnip, both of which she wants to eat, meaning the UCR (unconditioned response) is her salivation.
Emma also responds to stimulus generalization. When other's have food with a crinkly plastic bag, Emma will come running from her current napping spot to investigate. Things like chips, with louder plastic, illicit more of a response than a candy wrapper, which doesn't quite match the sound of her treats. This particular stimulus generalization can become quite annoying, especially when accompanied by several, whiny meows.