You shouldn't waste your parent's money on things.
Tom gives his son Billy a twenty dollar bill to get a gallon of milk at the store.
Billy wants to get a gallon of milk.
Billy buys the gallon of milk and takes the change straight home to his father.
Billy buys the gallon of milk and uses the rest of the change on junk food for himself.
Tom gives Billy five dollars for going to the store for him.
Tom yells at Billy for not bringing back all of his change.