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

[EX Q]: If two populations do not mate and produce fertile offspring, separate species result.
[EX A]: If two populations do not mate and produce fertile offspring, what results?

[EX Q]: The metallic bond is responsible for the properties of metals.
[EX A]: What is responsible for the properties of metals?

[EX Q]: Very low density is the concentration of gas molecules in the mesosphere.
[EX A]:
What is the concentration of gas molecules in the mesosphere?