It's rude to talk with your mouth full.
Finn was sitting in the cafeteria at school with his friends.
Finn wants to eat his food.
Finn starts to eat his food and chews with his mouth closed.
Finn puts his sandwich in his mouth and chews it with his mouth wide open.
Finn and his friends eat their food and talk outside afterwards.
Finn's friends are disgusted and no one wants to sit near him.