You will be given a definition of a task first, then an example. Follow the example to solve a new instance of the task.
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Earth rotates on its axis once times in one day.
Solution: How many times does Earth rotate on its axis in one day?
Why? The given output correctly poses a question based on the scientific fact given.

New input: The lower density of warm air causes it to rise above cold air.
Solution:
What property of warm air causes it to rise above cold air?