Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[EX Q]: Some compounds form rigid frameworks called crystals.
[EX A]: Some compounds form rigid frameworks called what?

[EX Q]: Processing of filtrate in the proximal tubule helps maintain the ph level in body fluid.
[EX A]: Processing of filtrate in the proximal tubule helps maintain what level in body fluid?

[EX Q]: A(n) increase in length happens to metal railroad tracks during the heat of a summer day.
[EX A]:
What happens to metal railroad tracks during the heat of a summer day?