"I am a bag of acid that dissolves what you eat. I expand to hold a full meal and shrink when empty. I growl when I want attention. What am I?"

Answer: The Stomach. The stomach contains hydrochloric acid strong enough to dissolve metal, which it uses to break down food. It can expand from about 75ml when empty to over 1 litre after a meal. The growling sound of a hungry stomach is caused by gas and fluid movement.