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

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


[Q]: Poles parts of the planet does solar energy have difficulty reaching.
[A]: What parts of the planet does solar energy have difficulty reaching?


[Q]: Metallic bonds hold together positive metal ions and their valence electrons.
[A]:
What types of bonds hold together positive metal ions and their valence electrons?