instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Deep currents are caused by differences in density at the top and bottom of the ocean.
answer:
What is caused by differences in density at the top and bottom of the ocean?


question:
Most ferns produce the same type of spores and are therefore called homosporous.
answer:
Most ferns produce the same type of spores and are therefore called what?


question:
Ph food enters the stomach after a period of fasting, a drop in what initiates the conversion of pepsinogen to pepsin.
answer:

When food enters the stomach after a period of fasting, a drop in what initiates the conversion of pepsinogen to pepsin?