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

[EX Q]: Gravity makes soil and rock particles move gradually down the slope during each thaw.
[EX A]: What makes soil and rock particles move gradually down the slope during each thaw?

[EX Q]: Evolution occurs by natural selection, a process whereby better-adapted members pass along their traits, according to darwin.
[EX A]: Evolution occurs by what process whereby better-adapted members pass along their traits, according to darwin?

[EX Q]: The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus.
[EX A]:
What is the main difference between eukaryotic and prokaryotic cells?