Write a correct answer to the given question based on its associated fact. Make sure that your answer is contained in the associated fact. Don't be creative and introduce any new word that is not mentioned in the associated fact! Remember that the associated fact has been rearranged to form the question. So, the correct answer words must lie within the associated fact. The correct answer can be a word, phrase, or even a sentence.

Fact: Water can make soil from rocks. 
Question: What causes rocks to be worn into soil?
Water.

Fact: When a population suddenly gets much smaller the effects are unpredictable. 
Question: What is the effect of a population suddenly getting much smaller?
it is unpredicatbale.

Fact: wind can be a requirement for plants to reproduce. 
Question: what can be required for plants to reproduce?

wind.