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.


[Q]: Fact: bears hibernate to conserve energy. 
Question: Why do bears hibernate?
[A]: conserve energy.


[Q]: Fact: Cells secrete acids and bases to maintain the proper pH for biological catalysts to work. 
Question: What secrete acids and bases to maintain the proper pH for biological catalysts to work?
[A]:
cells.