In Tibet it is considered good manners to stick out your tongue at someone as it is meant as a traditional form of greeting or sign of respect.
Tibetans used to believe that people with black tongues intended to poison somebody,and that devils had green tongues.By sticking out his tongue,a man is showing you that he isn't planning to poison you or he isn't a devil.So don't be alarmed if a Tibetan man sticks out his tongue when he meets you for the first time.