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

[EX Q]: The values of strangeness assigned to various particles are based on decay systematics.
[EX A]: The values of strangeness assigned to various particles are based on what?

[EX Q]: All enzymes have an optimal temperature and ph.
[EX A]: All enzymes have an optimal temperature and what?

[EX Q]: Muscles need stimulus to contract.
[EX A]:
What do muscles need to contract?