Why are everyone's teeth black in the movie Ghost Husband? Starring Mario

This is related to the betel nut culture in Thailand. There was no toothpaste and toothbrush in ancient times, so Thai people used betel nut to clean their teeth and freshen their breath. First, the betel nut, betel nut leaves and lime (the shell is burnt out) are mashed, mixed in a small vessel, then chewed in the mouth, and finally tobacco is added to brush your teeth. Eat areca dyed his teeth, first red and then black. In ancient Thailand, black wings were beautiful, but in modern times, it was banned as a backward folk custom. . .