Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
Example: Earth rotates on its axis once times in one day.
Example solution: How many times does Earth rotate on its axis in one day?
Example explanation: The given output correctly poses a question based on the scientific fact given.

Problem: The number of neutrons can vary between atoms of the same element.
Solution: The number of what subatomic particles can vary between atoms of the same element?