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: muscles contain cells. 
Question: What do muscles contain?
[A]: cells.


[Q]: Fact: expansion can cause fatigue. 
Question: what can cause fatigue?
[A]: expansion.


[Q]: Fact: A light turns on when the switch is open so the current can flow. 
Question: What happens when the light switch is open?
[A]:
The current can flow.