instruction:
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: 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: The gravitational pull of a natural satellite on Earth's oceans causes the tides. 
Question: The pull of what on the Earth's oceans cause the tides?
answer:
A natural satellite.


question:
Fact: Scales are used to measure mass of an object. 
Question: What can be used to measure mass?
answer:

scale.