Why do cats open their mouth's when they rub against you?

Just like dogs, cats drool. My dog drooled when it was expecting a treat. If you do this on a regular basis they get a reflex action anticipating the treat. Cats sense that you are going to give it a treat and it purrs, drooling awaiting the treat. I had a strange cat that did this with caramel candy. The cat liked caramel candy and at a certain time of day the cat would show up wanting the candy. Purring and drooling. It was a special habit.