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

Example Input: A penguin must do increase muscle mass before molting.
Example Output: A penguin must do what before molting?

Example Input: When light passes from one medium to another, it changes speed.
Example Output: When light passes from one medium to another, it changes what?

Example Input: Substances that cannot be broken down into simpler substances with different properties are called elements.
Example Output:
What are substances that cannot be broken down into simpler substances with different properties called?