Given a fact, create a question that can be answered using the fact. Construct the question such that it is unambiguous, has a unique answer and the answer can be given using the fact.

Fact: the looseness of soil increases the amount of oxygen in that soil
If soil is loose, it makes it what for oxygen to get in?

Fact: In the photosynthesis process oxygen has the role of waste product
Unlike CO2, oxygen is a waste product of?

Fact: lightning can cause a forest fire

In a forest without anyone around, a fire may be started by?