TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: Fractional crystallization is the separation of compounds on the basis of their solubilities in a given solvent.

SOLUTION: What is the separation of compounds on the basis of their solubilities in a given solvent?

PROBLEM: The bird ancestor that lived in trees developed wings, which is commonly associated with birds.

SOLUTION: The bird ancestor that lived in trees developed what feature commonly associated with birds?

PROBLEM: Millibar is the unit used to measure air pressure.

SOLUTION:
What is the unit used to measure air pressure?