Given the task definition, example input & output, solve the new input case.
Given a scientific passage and an answer, generate a question for the given answer.
Example: Passage: A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:. Answer: nucleotides.
Output: A frameshift mutation is a deletion or insertion of one or more of what that changes the reading frame of the base sequence?
The science passage defines frame mutation and the main compoment here is nucleotides. Hence the generated question is apt.

New input case for you: Passage: U-235 nuclei can capture neutrons and disintegrate more efficiently if the neutrons are moving slower than the speed at which they are released. Fission reactors use a moderator surrounding the fuel rods to slow down the neutrons. Water is not only a good coolant but also a good moderator, so a common type of fission reactor has the fuel core submerged in a huge pool of water. This type of reactor is called a light water reactor or LWR. All public electricity generating fission reactors in the United States are LWRs. Answer: moderator
Output:
What do fission reactors use to slow down the neutrons?