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.

Fact: mammals can generate heat by muscle action. 
Question: What can mammals generate heat with?
Muscle action.

Fact: mammals keep warm with hair. 
Question: what do mammals keep warm with?
hair.

Fact: steroids are chemical messengers that control many body functions. 
Question: what are chemical messengers that control many body functions?

steroids.