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: The mass of the Earth helps cause flowing rivers. 
Question: Which aspect of the Earth is needed to have flowing rivers?
answer:
Mass.


question:
Fact: fields make compasses point north. 
Question: What makes compasses point north?
answer:
fields.


question:
Fact: specialization occurs when animals and plants evolve to environmental conditions. 
Question: what happens when animals and plants evolve to environmental conditions?
answer:

specialization.