TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: Some compounds form rigid frameworks called crystals.

SOLUTION: Some compounds form rigid frameworks called what?

PROBLEM: The opening in the front of the eye is called pupil.

SOLUTION: What is the opening in the front of the eye called?

PROBLEM: If two populations do not mate and produce fertile offspring, separate species result.

SOLUTION:
If two populations do not mate and produce fertile offspring, what results?