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

PROBLEM: Most of the gray matter is on the surface of the brain, surrounding white matter.

SOLUTION: Most of the gray matter is on the surface of the brain, surrounding what other type of matter?

PROBLEM: Second is the smallest unit of time commonly based on.

SOLUTION:
What is the smallest unit of time commonly based on?