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

[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?

[EX Q]: Collision theory states that chemical reactions occur when atoms, ions, or molecules collide.
[EX A]: What states that chemical reactions occur when atoms, ions, or molecules collide?

[EX Q]: Biochemical reactions take place inside the organisms' cells.
[EX A]:
Where do biochemical reactions take place?