Two large spheres with the same diameter are mounted on vertical axles in a science museum. Both are coated in a thin layer of white plastic, and although they appear identical, one is actually a solid wooden sphere, while the other is a hollow metal sphere, constructed so that they have exactly the same mass. Because of the plastic coating, you can't tell which is which simply by touching. But you can tell by making them spin: which one will be harder to start spinning? Explain.