Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Example Input: Most of the gray matter is on the surface of the brain, surrounding white matter.
Example Output: Most of the gray matter is on the surface of the brain, surrounding what other type of matter?

Example Input: A crater can usually be found on the top of composite volcanoes.
Example Output: A crater can usually be found on the top of what kind of volcanoes?

Example Input: Mutation is essential in order for evolution to occur because it increases genetic variation and the potential for individuals to differ.
Example Output:
What phenomenon is essential in order for evolution to occur because it increases genetic variation and the potential for individuals to differ?