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

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

A change of season and less food usually cause certain animals to migrate.
Which two environmental changes usually cause certain animals to migrate?

Ovulation occurs before the endometrium thickens in estrous cycles.

What occurs before the endometrium thickens in estrous cycles?