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

[EX Q]: Oil is the most commonly used source of energy in the world.
[EX A]: What is the most commonly used source of energy in the world?

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

[EX Q]: Second is the smallest unit of time commonly based on.
[EX A]:
What is the smallest unit of time commonly based on?