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

[EX Q]: The mesopause is at the top of the mesosphere.
[EX A]: What is at the top of the mesosphere?

[EX Q]: Sodium chloride is an odorless white solid that is harmless unless consumed in large quantities.
[EX A]: What is an odorless white solid that is harmless unless consumed in large quantities?

[EX Q]: Density of matter reflects how closely packed the particles of matter are.
[EX A]:
What physical property of matter reflects how closely packed the particles of matter are?