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

Molar masses can be used to determine the mass of a quantity of material.
What can be used to determine the mass of a quantity of material?

The main advantage of urea is its very low toxicity.
The main advantage of urea is its very low what?

Solid -> liquid -> gas of these shows how frozen water changes as the temperature of the air increases.
Which of these shows how frozen water changes as the temperature of the air increases?