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.
--------
Question: Fact: Mammals generate heat mainly by burning calories. 
Question: Mammals generate heat mainly by what?

Answer: burning calories.


Question: Fact: Pollution is usually harmful to population size of a species. 
Question: What is usually harmful to the population size of a species?

Answer: Pollution.


Question: Fact: If a planet becomes too hot, then it cannot maintain a balanced ecosystem. 
Question: If a planet becomes too hot, then it cannot maintain what?

Answer:
balanced ecosystem.