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

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

In science, something that always applies under the same conditions is called a scientific law.
In science, what do you call something that always applies under the same conditions?

In a hydrogen replacement reaction, the hydrogen in the acid is replaced by an active metal.

In a hydrogen replacement reaction, the hydrogen in the acid is replaced by what?