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]: A theory is another name for a broad explanation that is supported by a great deal of evidence.
[EX A]: What is another name for a broad explanation that is supported by a great deal of evidence?

[EX Q]: The outer surface of the heart changes from positive to negative during depolarization.
[EX A]:
The outer surface of the heart changes from positive to negative during what?