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: birds fly using muscles in the chest. 
Question: What do birds use to fly?

A: Muscles in the chest.
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Q: Fact: nerves endings can feel cold. 
Question: What can nerve endings feel?

A: cold.
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Q: Fact: most canyons are formed due to gravity through erosion. 
Question: What force causes most canyons to form through erosion?

A:
gravity.
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