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

Most of the gray matter is on the surface of the brain, surrounding white matter.
Most of the gray matter is on the surface of the brain, surrounding what other type of matter?

If two populations do not mate and produce fertile offspring, separate species result.
If two populations do not mate and produce fertile offspring, what results?

The element sulfur is released into the atmosphere by the burning of fossil fuels.
What element is released into the atmosphere by the burning of fossil fuels?