This question touches on the philosophy of science. It might be answered with a discussion of the distinctions between the sciences. But there is really a lot of overlap in fields like physical-chemistry, bio-physics, and a number of other subfields. One wonders if the traditional three fields of science will maintain their distinction.
But you requested a simple statement. And physicists like to keep things simple. We ignore friction, or gravity, or assume that the geometry can be simplified. This separates the problem into simple components.
Why is sky blue?
What keeps the birds up?
Why is water wet?
Why does fire burn?
These are questions for which physics can offer some reasonable explanations; refraction, dispersion, gravity, forces, molecular cohesion, thermodynamic equilibrium. These answers cover a range of fields including optics, biology, chemistry, fluid dynamics, and many others. But most scientists think of the fundamental reason for these answers as the "physics" of the problem.
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