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.

[Q]: Fact: The mass of the Earth helps cause flowing rivers. 
Question: Which aspect of the Earth is needed to have flowing rivers?
[A]: Mass.


[Q]: 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?
[A]: Humans.


[Q]: Fact: Hormones are chemical messengers that are catalysts in the body. 
Question: What are chemical messengers that are catalysts in the body?
[A]:
hormones.