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:
Carbon dioxide is an example of a material that easily undergoes sublimation.

Ex Output:
Carbon dioxide is an example of a material that easily undergoes what?


Ex Input:
Birds have four limbs.

Ex Output:
How many limbs to birds have?