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

Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes.
Sodium-potassium pumps help nerve cells establish a voltage across their what?

Ionic compounds contain cations and anions rather than discrete neutral molecules.
Ionic compounds contain cations and anions rather than what?

Like the other outer planets, neptune has rings of ice and dust.

Like the other outer planets, neptune has rings of ice and what?