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.

[EX Q]: Fact: heat is detected by sensory nerve cells. 
Question: What is detected by sensory nerve cells?
[EX A]: heat.

[EX Q]: Fact: boulders are formed by snow, ice, and rock melting sometimes. 
Question: what melts sometimes to form boulders?
[EX A]: snow, ice, and rock.

[EX Q]: Fact: Heating a filament is helpful for seeing in the dark. 
Question: What is helpful for seeing in the dark?
[EX A]:
heating a filament.