TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: Do particles collide more in two reactants fluid they are both in fluid forms or solid forms.

SOLUTION: Do particles collide more in two reactants when they are both in fluid forms or solid forms?

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 energy content of foods is often expressed in calories.

SOLUTION:
Energy contents of foods are often expressed in what?