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

[Q]: A student should use a balance to compare the masses of two small rocks.
[A]: Which tool should a student use to compare the masses of two small rocks?

[Q]: The outer layer of the sun is made up of plasma.
[A]: What is the outer layer of the sun made up of?

[Q]: Melted iron do scientist's believe mercury's core is mostly made of.
[A]:
What do scientist's believe mercury's core is mostly made of?