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: humans use lipids to store energy. 
Question: what uses lipids to store energy?
humans.

Fact: Mineral graphite can pierce the skin surface. 
Question: What can pierce the skin surface?
Mineral graphite.

Fact: stimuli are detected by the optic nerve. 
Question: What are detected by the optic nerve?

stimuli.