It's rude to eat someone's food without their permission.
Mark was visiting at his friend Larry's house when he started to get hungry.
Mark wants to eat.
Mark asks Larry if he has anything to eat at his house.
Mark goes into Larry's kitchen and makes himself a sandwich when Larry is out of the room.
Larry goes into the kitchen and gets a bag of chips for Mark.
Larry is a little annoyed with Mark and asks him to leave at once.