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

Q: Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes.

A: Sodium-potassium pumps help nerve cells establish a voltage across their what?
****
Q: Millibar is the unit used to measure air pressure.

A: What is the unit used to measure air pressure?
****
Q: Eggs animals use incubation, how do they generally give birth to their young.

A:
When animals use incubation, how do they generally give birth to their young?
****