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

Q: After fertilization, a zygote forms into two cells.

A: After fertilization, how many cells does a zygote form into?
****
Q: A change of season and less food usually cause certain animals to migrate.

A: Which two environmental changes usually cause certain animals to migrate?
****
Q: Exotic species are also called invasive species because they threaten other species.

A:
Why are exotic species also called invasive species?
****