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

Input: Consider Input: A student should use a balance to compare the masses of two small rocks.

Output: Which tool should a student use to compare the masses of two small rocks?

Input: Consider Input: Gabbro is a dark dense rock that can be found in the oceanic crust.

Output: Gabbro is a dark dense rock that can be found in what?

Input: Consider Input: Compounds with metals and nonmetals sharing electrons are known as ions kinds of compounds.
Output: Compounds with metals and nonmetals sharing electrons are known as what kinds of compounds?