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

Example input: Earth rotates on its axis once times in one day.
Example output: 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.
Q: Ph food enters the stomach after a period of fasting, a drop in what initiates the conversion of pepsinogen to pepsin.
A:
When food enters the stomach after a period of fasting, a drop in what initiates the conversion of pepsinogen to pepsin?