What should be strongly forbidden and yet not realized by some well-meaning people is using their chopsticks to pick up food for someone near them. Nobody likes eating something touched by chopsticks that have gone into another person's mouth several times!
Eating Manners
Nobody wants to see the shape or colour of food in your mouth, especially when it is being mixed with saliva, so do not open your mouth as you chew or eat. For the same reason, it is just as bad to speak when there is food in your mouth, and also because doing this makes your voice sound ‘unclean’.