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

Example Input: Diffusion process helps absorptive protists absorb food molecules across their cell membranes.
Example Output: Which process helps absorptive protists absorb food molecules across their cell membranes?

Example Input: Very low density is the concentration of gas molecules in the mesosphere.
Example Output: What is the concentration of gas molecules in the mesosphere?

Example Input: Ions that contain fewer electrons than protons have a net positive charge and are called cations.
Example Output:
Ions that contain fewer electrons than protons have a net positive charge and are called what?