Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
--------
Question: A change of season and less food usually cause certain animals to migrate.

Answer: Which two environmental changes usually cause certain animals to migrate?


Question: In the tropics tradewinds are the prevailing winds called.

Answer: In the tropics what are the prevailing winds called?


Question: A fact or question is only considered science if it is testable.

Answer:
A fact or question is only considered science if it has what property?