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: predicting weather requires meteorology. 
Question: What requires meteorology?
[EX A]: predicting weather.

[EX Q]: Fact: When heat passes through metal, that metal expands. 
Question: What happens to metal when heat passes through it?
[EX A]: It expands.

[EX Q]: Fact: Active immunity results when an immune response to a pathogen produces the basis of immunological memory. 
Question: Active immunity results when an immune response to a pathogen produces the basis of what?
[EX A]:
immunological memory.