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

Example Input: The main advantage of urea is its very low toxicity.
Example Output: The main advantage of urea is its very low what?

Example Input: Water, air, and milk list includes only examples of matter that can take the shape of a container.
Example Output: Which list includes only examples of matter that can take the shape of a container?

Example Input: Deeper layers of rock must be older than layers closer to the surface, this is the law of superposition.
Example Output:
Deeper layers of rock must be older than layers closer to the surface, this is the law of what?