instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Collision frequency is greater for homogeneous category of catalysts, which also tend to be more sensitive to temperature and more 'expensive'.
answer:
Collision frequency is greater for what category of catalysts, which also tend to be more sensitive to temperature and more 'expensive'?

question:
Exotic species are also called invasive species because they threaten other species.
answer:
Why are exotic species also called invasive species?

question:
Ground tilting is caused by the buildup of stress in the rocks.
answer:
What is caused by the buildup of stress in the rocks?