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

Ex Input:
Producing sperm and secreting testosterone are the main functions of male reproductive system system.

Ex Output:
Producing sperm and secreting testosterone are the main functions of what system?


Ex Input:
Line graphs are especially useful for showing changes over time.

Ex Output:
Line graphs are especially useful for showing changes over what?


Ex Input:
Excessive bending stress usually causes fractures on a bone.

Ex Output:

What usually causes fractures on a bone?