instruction:
Write a correct answer to the given question based on its associated fact. Make sure that your answer is contained in the associated fact. Don't be creative and introduce any new word that is not mentioned in the associated fact! Remember that the associated fact has been rearranged to form the question. So, the correct answer words must lie within the associated fact. The correct answer can be a word, phrase, or even a sentence.
question:
Fact: heating salt water leaves the salt on the surface. 
Question: heating salt water leaves the salt where?
answer:
on the surface.


question:
Fact: Playing is just one of many ways that humans and other animals learn how to behave. 
Question: What uses playing as just one of many ways to learn how to behave?
answer:
Humans.


question:
Fact: Antigens are molecules that the defense system recognizes as foreign to the body. 
Question: What are molecules that the defense system recognizes as foreign to the body?
answer:

antigens.