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

[EX Q]: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only contract.
[EX A]: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?

[EX Q]: If two populations do not mate and produce fertile offspring, separate species result.
[EX A]: If two populations do not mate and produce fertile offspring, what results?

[EX Q]: In science, something that always applies under the same conditions is called a scientific law.
[EX A]:
In science, what do you call something that always applies under the same conditions?